"It was working yesterday, I swear"

My favorite example of this was some code that would always fail in production, but test out fine for devs.

Turned out the developer had used static variables to communicate between runs of dynamically loaded modules, which sadly, you get different sets of when you are loaded onto multiple processors in the production environment, versus single instance testing by the dev concerned.

sometimes you get this illusion, but

the case is always the same - it wasn't working properly yesterday ;p