Are you familiar with chess? What classes/objects and methods/functions would you use to implement the game of chess? Who knows if checkmate is reached? What about timed games?

It always leads to an interesting discussion and you can easily throw out prodding questions if they get stuck. I've never had anybody give me the exact same answer, and it's been complimented post-interview a handful of times.

How much of a red flag is it if a programmer gets concepts like interfaces and classes, but when asked what an interface is, can't give me a straight answer?