A favourite question is "What's the longest you've ever sat in front of a computer for?" randomly thrown into the flow. I have yet to meet a trully gifted programmer who has not pulled the odd etreeeeeemly long session working on a particularly fascinating problem..... BTW I don't advocate extended hours as a good practice as we are all aware how it can quickly sap morale and diminish productivity.
I like this because candidates generally don't have a pat answer so you can assess enthusiasm for what we do, honesty in answering, depth of geekdom, and the ability to think on their feet all in about 60 seconds.
The responses to this seemingly simple question are quite fascinating.
In reply to Re: Number 1 mistake to not avoid during an interview