Though it's unlikely your date will reveal his or her deepest darkest secrets too early in the relationship, this question does present an interesting challenge. Does she have a weird habit that you never would have guessed? What about a secret talent that makes him unique? This question can be exciting because it's likely neither of you will be sure just how much to reveal. So have fun with it (without freaking your date out too much).
But remember -- while this can be a fun question to make your date squirm, be prepared for the question to be thrown right back at you!