"Best Catch" and "One That Got Away" are the only good questions here, and "Best Catch" is the only one I ask every single time.

This is one of the most important question because it reveals the "soul" of the tester; the flavor of brokenness that excites them most.

For every once in a lifetime "you won't believe it, but..." answer there are dozens of boring "one time mysql went down on prod" answers. It's not a golden snitch or anything like that, but I find it to be indispensable in the interview process.