Personal favourite.
From a previous existance and left for our entertainment by a programmer who had long since left the company concerned.
Green-screen app with what can only be described as a shed-load of input capable fields on the particular screen in question. All are validated against reference data and each other and any in error are highlighted and some nice scrolly messages appear at the bottom of the screen telling you about all the errors.
One day a new user calls saying that her screen is locked out, we ask what she was using and swear (offline) when she tells us. The particular screen in question had a few sensitive things open for update. So, fact-finding time:
"What's the screen number?"
"It hasn't got one."
"No, look in the top left corner of the screen, there's a screen number."
"No there isn't."
Hmmmm, strange.
"What does it say on the screen?"
".......I'd rather not say."
Hmmmmm, very strange. A quick look at the user's job reveals a screen display member that we don't immediately recognise. Do we a) pull the source to see what it looks like or b) exit, en masse, to the user's desk to see what's up? b) wins out and off we go.
Turn out that the new user has managed to get every single field invalid in some way or another and a long-unused error routine has creaked into life. There, on the screen, with all the keys disabled, resplendant in HUGE block letters lovingly constructed out of individual spaces displayed inverse, highlighted and blinking is one word:
BOLLOCKS!