Some people think I'm very mean; others think I'm very nice. Neither realise that one can be both depending on the person to whom they're talking, and are surprised when they see me acting contrary to their impression. If you ask me, I'd say I'm a hardcore tsundere.
EDIT: Also, people automatically presume I'm atheist because I'm smart, which I find hilarious; as such presumptions are akin to admitting that only fools follow organised religion. (I'm Deist)