Ad Hoc Testing: The Secret Weapon for Finding Hidden Bugs

Updated April 2025

Ad hoc testing is done when you need to identify an issue quickly or when you want to trigger unexpected bugs. For instance, you might have received a bug report from a customer and now need the steps to reproduce it. Or you might want to perform some exploratory testing on a new feature. Ad hoc testing is unplanned, but you still need to follow a plan when you do it. Ad hoc(adjective) made or happening only for a particular purpose or need, not planned before it happens. (Cambridge Dictionary online)

What is Ad Hoc Testing?

Generally, you should carefully plan and execute all your tests. There are good reasons for this. Tests need to be thought through to ensure they are touching all the moving parts of the code. They need to be executed in the correct sequence, with the correct starting conditions. All steps must be documented and the results are written down.  However, formal testing such as this often misses important bugs. This is because, in general, you are only testing the expected user journeys through your application. But users are all too good at doing some unexpected things. This is why users are so good at finding bug in your production code. So, what do you do when you can’t rigorously plan your tests? This is when ad hoc testing is needed. You use ad hoc testing to try and find ways to break your application. It is almost the exact opposite of the usual strictly planned and executed testing you are used to. Some people use the term “monkey testing” since ad hoc testing is like monkeys randomly pressing buttons. However, this is an unfair characterization. While ad hoc testing is unplanned, it is focussed. A skilled ad hoc tester will know ways to trip systems up and will try all these tricks to cause the system to fail.

Who Performs Ad Hoc Testing?

Anyone involved in the software development process can conduct ad hoc testing. This includes developers, testers, business analysts, and even product managers. However, experienced testers or QA professionals are best suited for this type of testing. They have a deep understanding of the system and know where hidden bugs might be lurking. Their ability to think outside the box makes their ad hoc testing more effective.

When to Execute Ad Hoc Testing?

You can execute Ad Hoc testing when there is limited time, but you still need to find critical issues. It is done after formal testing to catch unexpected bugs. Here are some key stages where ad hoc testing can be useful:

• Early Development Stages:
  
  • Helps identify major issues before formal testing begins.
  • Provides developers with quick feedback on their code.
• During System Integration Testing:
  
  • Exposes hidden defects when different components interact.
  • Helps uncover integration issues that structured testing may miss.
• After Formal Testing:
  
  • Serves as a last check to catch missed defects.
  • Ensures the system is robust before release.
• In Response to User Feedback:
  
  • Helps recreate and investigate reported issues.
  • Provides quick insights to fix unexpected errors.

When Not to Conduct Ad Hoc Testing

While ad hoc testing is useful, it is not always appropriate. Here are some cases where it should be avoided:

• When Test Cases Are Incomplete:
  
  • If test cases are incorrect, they should be fixed before running any ad hoc testing.
• During Beta Testing:
  
  • Beta testing focuses on structured feedback from real users.
  • Unplanned testing can interfere with beta results.
• For Critical Systems Without Logs:
  
  • If no logging or tracking is available, ad hoc testing may fail to capture crucial details.

Using Ad Hoc Testing in Different Development Phases

Ad hoc testing is not recommended in the planning phase since there is nothing to test yet—requirements are still being gathered. During development, however, it becomes useful. Developers can quickly test their code before formal testing begins, catching obvious defects early and preventing them from carrying over to later stages.

The testing phase is the best time for ad hoc testing. By this point, most features are in place, and structured testing is underway. Ad hoc testing acts as an extra layer of defect detection, finding unexpected issues that formal test cases might overlook. It is particularly effective at exposing edge cases and unpredictable system behavior.

After release, ad hoc testing helps diagnose production issues. When users report bugs, testers and developers use ad hoc methods to quickly reproduce and analyze the problem.

Decoding Ad Hoc Testing: An Informal Approach

It might seem like ad hoc testing is just a waste of time. Why would you ask a tester to go out of their way to break your system? Or if you receive a bug report, why do you need to do anything other than verify it was correct?  Once upon a time, there was a QA tester working on a taxi-booking app in London. Tony was a skilled tester with years of experience. This taxi app was passing all its QA tests with flying colors. But taxi drivers testing the beta version were triggering an occasional bug that caused the app to crash. The app was designed to recover elegantly from crashes, so all the drivers were oblivious. But clearly there was an issue. Tony decided he would try some ad hoc testing to see if he could replicate the bug.

Advantages of Embracing Ad Hoc Testing

With ad hoc testing, you can unlock several advantages for your team:

• Rapid Identification of Critical Defects: Ad hoc testing helps you find critical bugs fast. You’re testing in real time, so issues with new features or changes pop up immediately.
  
• Enhanced Creativity and Critical Thinking: Without strict scripts, ad hoc testing lets you get creative. You explore the app in unexpected ways and catch bugs that structured tests might miss.
  
• Cost-Effectiveness and Flexibility: It’s cost-effective because it doesn’t need lots of resources or preparation. Plus, it’s flexible—you can jump in and test at any stage, no matter what changes come up.

Key Types of Ad Hoc Testing

Monkey Testing: Embracing Randomness

Randomly interacting with the application to check its response to unexpected inputs. Testers enter random data, click buttons, or stop processes to look for crashes or unusual behavior. While unstructured, it can uncover vulnerabilities that structured tests might miss.

Buddy Testing: Collaborative Defect Detection

A developer and a tester evaluate a module together. They share feedback and insights to identify defects more efficiently. The developer provides technical knowledge, while the tester offers a user-centric perspective.

Pair Testing: Dual Perspectives for Deeper Insights

Two testers work together on the same application. One tester interacts with the system, while the other documents findings and suggests test ideas. This method increases test coverage and helps catch defects a single tester might miss.

Ad Hoc Testing vs. Smoke Testing

Smoke testing checks basic functionality on a new build to ensure stability before detailed testing. It follows predefined test cases. Ad hoc testing is unstructured and used when there’s limited time or missing documentation. While smoke testing ensures stability, ad hoc testing finds unexpected issues.

Ad Hoc Testing vs. Exploratory Testing

Both methods allow flexibility, but exploratory testing has more structure. Ad hoc testing is fully unstructured, focusing on quick defect detection. Exploratory testing is flexible but follows a loose structure and documents test findings, with an emphasis on learning while testing.

Strategic Implementation of Ad Hoc Testing

Ad hoc testing works best when applied at the right time with a focused approach. It is most useful when formal testing is complete, but there is still a need to catch missed defects. It also helps when time is limited, and structured testing is not an option.

Optimal Timing: When to Employ Ad Hoc Testing

• After Formal Testing: A final sweep to catch unexpected bugs before release.
• During Tight Deadlines: A fast way to uncover issues when time is short.
• When Investigating Reports: Helps recreate and diagnose user-reported bugs.
• For High-Risk Areas: Identifies flaws in critical application components.

How to Implement Ad Hoc Testing Effectively

• Leverage Experienced Testers: Skilled testers know where to look for hidden bugs.
• Focus on High-Impact Areas: Prioritize core functionalities and critical workflows.
• Use Real-World Scenarios: Mimic how actual users interact with the system.
• Combine with Other Methods: Pair ad hoc testing with structured approaches for best results.
• Document Key Findings: While ad hoc testing is unstructured, noting major issues helps improve fixes.

In the real-life story above, Tony began by trying to replicate conditions where a taxi driver would use the app. He had an instinct that the problem was related to the GPS signal (a frequent problem in London’s urban canyons). Added to that was the fact the taxis were constantly moving, meaning they were changing cellular transmitter frequently, and sometimes losing data completely.  To replicate these conditions, Tony started testing the app during his commute to and from work on the bus. After a few journeys, he was able to trigger the bug. But as this was ad hoc testing, that was only of limited use. Tony knew now how to trigger the bug, but he still didn’t know the root cause. So, he turned to the developers for help. He asked them to instrument the GPS and log its location constantly. Tony repeated the testing with this special build of the app. This time, when the bug recurred, he was able to see the GPS track. At many points the phone lost GPS position. Usually, this was fine, but, occasionally, it would pick a really arbitrary position in the middle of the river Thames. This was causing the application to panic as it made no sense, and it would crash and restart. This data allowed the developers to fix the bug.

Key Considerations for Effective Ad Hoc Testing

Here are some key considerations to keep in mind for effective ad hoc testing.

1. Put yourself in your user’s shoes. One of the key skills in ad hoc testing is understanding how a user might accidentally (or maliciously) misuse an application. This comes with experience. Knowing how existing bugs were triggered is vital. It also pays to understand UX/UI principles.
2. Leverage tester expertise and system knowledge. Experienced testers bring deep system knowledge, allowing them to identify weak points and areas prone to failure. Understanding the architecture and previous defects helps in spotting potential issues quickly.
3. Follow your instincts. Many of the best ad hoc testers develop a 6th sense for the sort of things that may trigger a failure. You may notice that the application “stutters” when you perform a certain sequence of actions. It may respond a bit slower or briefly become unresponsive.
4. Balance with structured testing methods. Ad hoc testing should not replace structured testing but complement it. Use it as a final check after formal testing to catch defects that scripted tests might miss.
5. Try to think of external factors. Try to think of things that may influence how the application behaves. For instance, all the bad things that might happen when the connection switches between WiFi and cellular data. Or what happens when the backend has a failure.
6. Focus on high-risk areas. Prioritize testing critical functionalities where failure would have a significant impact. Areas prone to performance issues, security vulnerabilities, or frequent updates are prime candidates.
7. Use logs and instrumentation where possible. As the example above shows, sometimes you need to look at the actual data in an app to identify a problem. Other times, logs will allow you to spot unexpected responses. And instrumenting the UI may reveal things like potential race conditions.
8. Integrate with formal testing strategies. Ad hoc testing should be part of an overall testing approach. Use insights gained from exploratory testing to refine automated and manual test cases.
9. Be rigorous. You need to follow any test to the end.  You need to try and test all possible combinations of actions to make sure you trigger any bugs.
10. Keep track of what you did. Closely related to the above point is the need to record and track exactly what you did. This will be essential to identify the precise steps to reproduce any bug you trigger.
11. Explore lesser-tested areas.  Shift your focus to parts of the application that are rarely tested. Features that have seen minimal structured testing may hide unexpected issues.
12. Test under different conditions.  Try running the application on various devices, operating systems, and network environments. Changes in hardware or connectivity can reveal critical bugs.
13. Time your testing sessions.  Ad hoc testing is best done in short, focused bursts. Too much random testing at once can lead to overlooked insights.
14. Share insights with the team.  Even though ad hoc testing is unstructured, discussing findings with developers and other testers helps refine testing strategies and improve issue resolution.

7 Ad Hoc Testing Examples

Here are some examples of ad hoc testing where unexpected paths are explored to uncover hidden issues:

1. Randomly clicking on different buttons and menus in an app to see if any lead to unexpected behaviors.
2. Interrupting a page load by switching tabs or applications midway to check if the browser handles interruptions gracefully.
3. Testing an app by entering unusual data formats or extreme values into form fields to see how it handles edge cases.
4. Switching between different user accounts while interacting with the app to identify issues with session management.
5. Forcing the app to reload multiple times in quick succession to test how it manages repeated refresh actions.
6. Toggling network settings, switching between Wi-Fi and cellular, to see if the app behaves properly in varying network conditions.
7. Performing actions at random intervals, like rapidly pressing buttons, to detect potential responsiveness issues.

Ad Hoc Testing Mistakes to Avoid

You need to plan your unplanned testing. It really isn’t about randomly pushing buttons to see what happens. Rather, an ad hoc tester is trying to think of all the unlikely things that a user might do. Or using their knowledge of the app architecture to try and trigger exceptions and unknown states. However, there are some common mistakes to avoid:

1. Relying solely on ad hoc testing for regulated environments: In industries like healthcare or finance, ad hoc testing may not meet the stringent compliance requirements. These industries often demand repeatable, documented testing processes, which ad hoc testing lacks. Ensure formal testing is incorporated where necessary to meet regulatory standards.
   
2. Using ad hoc testing for mission-critical systems: For systems where a failure could have serious consequences, such as those used in healthcare or aviation, ad hoc testing alone isn’t enough. You must implement formal testing methodologies, with clear test cases and validation steps, to ensure safety and reliability.
   
3. Overlooking structured testing in complex projects: In large-scale projects with extensive functional and non-functional requirements, ad hoc testing may not provide comprehensive coverage. A mix of structured testing, including planning, scripting, and automation, is essential to fully test complex systems.
   
4. Skipping consistency for regression testing: Ad hoc testing is unscripted and spontaneous, which can be an issue when consistency is crucial. For regression testing, where the same tests need to be run repeatedly, automated or scripted tests are more effective at ensuring accuracy and repeatability.
   
5. Neglecting test documentation: Ad hoc testing may not produce the detailed documentation needed for traceability, reporting, or compliance in some projects. In such cases, formal test cases with clear documentation are necessary to meet organizational or regulatory requirements.

Challenges for ad hoc testing

Normally, you need skilled manual testers for ad hoc testing. Ad hoc testing finds bugs that no other approach can. But manual ad hoc testing is time-consuming and slow. You need to be thorough, meaning testing as many combinations of actions as possible. For instance adding and deleting items from a cart multiple times and in different orders. This can rapidly become quite a mechanical process. When your ad hoc testing starts to become repetitive, you might be better off creating automated tests. Systems like Functionize Architect make this really easy to do.  Another important challenge is recreating real-world conditions. As Tony’s story above shows, the real world can pose challenges that don’t exist in normal test environments. There are several key differences:

• The backend is usually different. Even when the setup is identical, your production backend has dirty state.
• The operating environment is different. This is really true for mobile apps. However, it is increasingly true for all apps with the rise of responsive UIs.
• Your test devices are probably very different to real user devices. E.g. you probably have maintained them, performed security updates, rebooted them frequently, etc.

You can only solve these challenges with rigorous ad hoc testing (if that isn’t a contradiction!)

The Future of Ad Hoc Testing in Agile Environments

As Agile development moves faster, ad hoc testing is evolving too. AI and automation are stepping in to help testers keep up with the speed and complexity of rapid release cycles. Even though ad hoc testing is usually spontaneous and informal, these tools are making it possible to do more, faster, without losing the quality you need.

Adapting to Rapid Development Cycles

With development cycles speeding up in Agile environments, you need to be quick in finding bugs. Ad hoc testing is crucial in these scenarios, but it’s also where automation can step in to make things more efficient. Tools powered by AI can help set up test environments and generate clean test data quickly, cutting down the time you spend on repetitive tasks. This way, you can focus on exploring the app in the unstructured, intuitive way ad hoc testing requires.

Evolving Tester Roles and Responsibilities

As automation tools handle repetitive tasks, your role as a tester is shifting. Instead of focusing on running the same tests over and over, you’ll be diving into more complex scenarios, looking for issues that tools might not catch. AI will give you valuable insights, helping you spot patterns and make faster, smarter decisions. This shift means you’ll spend less time on manual tasks and more time exploring the application, discovering bugs where automated testing can’t reach.

Conclusions

• Ad hoc testing is effective at identifying bugs that are hard to reproduce.
• It helps test the stability of your system against malicious or improper use.
• Ad hoc testing increases the overall test coverage of your product.
• It is a crucial part of any testing strategy, though it can be resource-intensive and expensive.
• Using Functionize Architect, you can automate parts of your ad hoc testing.
• Despite automation, some manual testing will always be necessary.

FAQs on Ad Hoc Testing

What tools can assist in conducting ad hoc testing?

Even though ad hoc testing is spontaneous and unstructured, there are a few tools that can make it easier. 

• Functionize Architect can help automate some parts of your ad hoc testing, so you don’t have to do everything manually. 
• Selenium lets you manually interact with web applications, which can be really helpful in testing unexpected behaviors. 
• Cucumber, though mostly for behavior-driven development, can also be useful when you want to write and execute tests in a more structured way using plain language.

What are common misconceptions about ad hoc testing?

A big misconception about ad hoc testing is that it’s just randomly clicking around and hoping to find bugs. In reality, successful ad hoc testing requires a lot of thought. Testers use their knowledge of the app to think about unusual user behaviors and try to trigger hidden bugs. Another misconception is that it’s all about testing with no documentation. While it’s informal, keeping track of your findings is important. You want to be able to revisit what was tested and what issues were found.

How can testers document their findings during ad hoc testing if there’s no test plan?

Even though ad hoc testing doesn’t follow a specific plan, documenting your findings is still important. You can use session-based test management tools to keep track of what you tested, the steps you took, and any bugs you discovered. Another option is to simply take notes during the testing process, outlining the actions you performed and the results you observed. This way, even though your testing is informal, your findings remain well-documented and can be easily referenced later.