I have this weird thing on Facebook where I get confused if two people have the same starting letters in their first and last names. So, like, my eyes see Mark Edwards, but my brain translates it into Mike Eddington (two made-up names, by the way). But it's happened in the case where the first guy is someone I know only very casually and the second is someone I'm pretty good friends with, and I end up getting into a strange Facebook debate with the casual acquaintance thinking he's the good friend, until I realize my mistake and then apologize.
tl;dr: I'm weird.