A while ago a friend who I hadn't seen since 2011 messaged me on Facebook asking if I could take a look at his account on a guitar forum we both used to frequent, because he thought his account was stolen, but couldn't check it for activity without making a new one(and probably also having enough posts to be considered "in").

So I asked who sold him that Music Man Petrucci 7-string in 2009(or so) and after he answered correctly, I followed through with his request.

My point being: it helps to have such a question prepared for everyone - or alternatively - a question concerning some fact about you that only people who know you could answer.