I found a bug that I can't reproduce, what do I do?
This type of situation can be very frustrating both for the tester and the developer. Some bugs only occur if a specific series of actions happen at the same time, and you may not be aware of what you did to make that bug appear.
There are many ways to try to get the bug to appear again such as using video recording, or screenshots, but if you are completely unable to reproduce the bug, filing a bug report with as much detail as you can is still worthwhile, as the developer may have an idea of where the bug comes from and what can be done to fix it.
Here is an interesting article for more information on what to do when you can’t reproduce a bug.