Yesterday I sent a facebook friend request a to a friend of mine from elementary school. This evening he sent me a message saying he didn't recognize me because I look like a boy in my profile picture. Made my day!
That's pretty awesome. What makes it better is that you didn't ask for it, it just came naturally.
My mom dug up an old picture of mine (pic of me @ 12) and said I looked like a girl when I was a kid, and I thought to myself 'I miss those days'.