What is Intermittent issue in Testing? How do we find it?
The simple answer is: an issue that occurs at an irregular interval or not in a continuous or steady manner.
Another way to define this "a mysterious and undesirable behavior of a system, observed at least once, that we cannot yet manifest on demand." - James Bach.
What are some examples of this?
In my experience, race condition issues come to mind as these can look intermittent when they are found.
A race condition, by definition, is dependent on a sequence of events or timing of events. If an event/process happens out of order or a secondary process finishes first where the results are dependent on the first process finishing, a bug occurs. This looks intermittent due to the user/tester not being aware of the order of events that need to occur.
Another way this occurs is due to a caching issue. When one person has an issue that can't be replicated by another, we often tell that person to "clear their cache/cookies" and the problem goes away. This is one of the simplest explanations and we usually don't expound upon it afterwards.
Because software is complex, there are lots of reasons why intermittent issues occur: hardware going bad, lack of CPU or memory resources, a 3rd party dependency is going awry, etc.
Just because an issue happens once doesn't mean it was a fluke and is unlikely to occur again. Be cautious of when these occur as it's likely to occur again.
The easiest definition of the intermittent issues in testing could be unexpected random errors that testers have to deal with. Most of the time, these issues are triggered because of unanticipated reasons.
Also, the intermittent issues can also be understood as inconsistent behavior of the application for multiple test efforts made on a build while sticking to the same process. All in all, it is more an error or bug that occurs in a short time and not repeatedly
Usually all the tester in the outsourced software testing companies facing this common issues while testing an application.
Intermittent issues or Bugs are the type of bugs which are not having consistent behavior. Means if you execute same test multiple times on any application, each time it will give you some different result. It doesn't means that these issues are not a defect, but it is very difficult for tester to provide the exact steps in order to reproduce this issue.
What we can do only is : Write the exact steps of testing with the Test Data entered, then note the final result of 1st testing, if possible, attach the screen shot or create video recording. Do the same 2nd time and write the result of 2nd time, also screen shot. So like this we can get some results with us and based on that we can report the bug.