My favorite technical interview question is to ask the candidate about the worst bug they ever wrote. I let them choose their own definition of "worst", and see if they choose something that was hard to debug, or caused a lot of trouble or was just something they thought was a dumb mistake.

It usually provides them with an opportunity to talk technically about something they know and helps me understand how well they communicate problems and solutions. Plus, it's sometimes fun to hear the stories.

My own personal worst bug (where by worst I mean "had the worst impact") was when we disabled a large chunk of the southern Beijing cell phone system for a short time during the night whilst deploying a field test of new base station hardware. That was a stressful rollout.