Long but hilarious:
I was deeply concerned about how we have a single, non-paginated call to a backend service, returning hundreds of entries, which has to be enriched with constant data fetched from our db for each entry. FOR EACH ENTRY. AND FOR EACH REQUEST.
I voiced my concerns to my PM, who called me a "rage prophet" for it.

As expected, the call took 20-something seconds to complete.
Ten minutes before the CEO comes over to have a look, another dev changes his loosely-related service, and the entire super-heavy, sprawling abyss of enrichment pipeline returns in sub-second timing!!!

CEO: guys, this is too fast. You have to slow it down a bit. It doesn't seem reliable that we're able to get all this data immediately.
PM: you see, rage prophet, it all worked out in the end
Me: #$@%$&!!!!!