The bug is not reproduced ... What to do?

Under the cut, my speech at the Fun ConfeT & QA conference on the topic “The bug is not reproduced ... What to do?!”

You have found a bug - but you cannot reproduce it.
You found a bug, it was successfully reproduced - but the next day you can no longer reproduce it.
You found a bug, it is successfully reproduced - but only on your machine, while on others everything is working fine.
You have found a bug, it is successfully reproduced - but not on the developer’s machine and it cannot fix it.
You found a bug, it was successfully reproduced, and now it disappeared by itself, although the developers say that they did not fix anything.
Familiar? For sure.
What to do in such situations?
To write in a bug tracker or not to write?
Was there a bug at all? Will you believe it?
How much time to spend on trying to reproduce a cunning bug?
I will tell you my rules and little tricks, how to act in these cases.