The technical term is Heisenbug: https://en.wikipedia.org/wiki/Heisenbug

Did you perform the correct incantation?

Check the global temp vars, files and caches. Encapsulated scoping is a lie we've been told to help us sleep better at night. Some of those seemingly well defined system functions bleed out to caches for (slight) performance gains.

You could have the most locked down explicitly defined well scoped code in the world, and it just takes one measly baked in call to a global function to shatter that dream house into a thousand pieces.

I'm looking at you string-match; you know what you did.

Bugs wants to be nice, they see how stressed we are so they love playing hide and seek to relax us XD

What was the cause?

Have you tried turning it off and on again?

Some bugs only trigger on the first run. Or the second. Or the third. Or...

When I find bugs, I use screen capture software to repo it.

Got introduced to the phrase "your tongue was in the wrong spot in your mouth", for something that can't be reproduced.