> As well as the closely related 'that can't possibly have ever worked' and sure enough it never did.

I had one of those, customer is adamant latest version broke some function, I check related code and it hasn't been touched for 7 years, and as written couldn't possibly work. I try and indeed, doesn't work. Yet customer persisted.

Long story short, an unrelated bug in a different module caused the old, non-functioning code to do something entirely different if you had that other module open as well, and the user had disciverdd this and started relying on this emergent functionality.

I had made a change to that other module in the new release and in the process returned the first module to its non-functioning state.

The reason they interacted was of course some global variables. Good times...

Global variables... the original sin if you ask me. Forget that apple.