Half way through the interview, I had an epiphany. I really didn't want to work there. It was cultural, it just wasn't going to fit.
I didn't waste any more time. Half-way through a white-board challenge, I put down the marker and said, plainly, "okay, I've seen enough, I don't want to work here - thanks and let me not waste any more of your time", picked up my coat and left.
It wasn't a bad interview. It wasn't a terrible one. Nor was it because of the whiteboard question, or anything like that.
I just didn't like the guys. That's all it was. And I couldn't stand the idea of working for them - just the way the interview proceeded. I don't need to give details.
It was really the only time I ever got up mid-interview and left.