What was the cause?

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.

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

Did you perform the correct incantation?

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

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

Have you tried turning it off and on again?

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