Today there was a question on SO, where the author was given an NP-complete problem during an interview and he obviously hadn't been told that it was one.
What is the purpose of asking such questions? What behavior does the interviewer expect when asking such things? Proof? Useful heuristics? And is it even legitimate to ask one if it's not a well-known NP-complete problem everyone should know about? (there's a plenty of them)