The bug does not play ... What should I do ?!

Under the cut my speech at the conference Fun ConfeT & QA on the topic “The bug does not play ... What should I do ?!”

You found a bug - but you can’t 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 successfully reproduces - but only on your machine, but on others everything works fine.
You 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 didn’t fix anything.
Is that familiar? Surely.
What to do in such situations?
To write to the bug tracker or not to write?
Was there a bug at all? Would they believe you?
How much time should I spend trying to reproduce a tricky bug?
I will tell you my rules and little tricks how to act in these cases.