The one I always ask is:

What's the worst mistake you ever made at work and what did you do about it?

And I expect something pretty bad from anyone who's innovative or star material. I expect the type who has difficulty with integrity to not have any answer and dance around it till something lame occurs to him or her.

There is no right answer, of course. It's a chance to give a short character essay on oneself. The best replies I've heard for it tended to involve the most catastrophic kinds of mistakes. How the candidate discuss it and its result is the meat.

Update: Oh, ++, of course for the OP; and my own worst was pretty bad and I paid for it with an 85hr week and a crash course in Oracle queries; you won't get the story because even with the successful penance quite a few customers paid for it too.