Back in college a friend of mine managed to get an "Unexpected operator IQ error" while compiling some Eiffel code. I'd earlier managed to get remote write access to his terminal and popped that one onto his screen to see just how frustrated he was getting. He blew his top.

Another one I've created was in an administration shell. The prompt looked like a normal unix prompt so people invariably ended up typing 'ls' in it. To which it would reply, "You're not in unix any more Toto."