People who assume I'm rich. I'm not. My dad is. Difference there is that I get nothing, not even in the will. I have to work to get my money, and when I get a job, I'll be fine with it.
People who assume I'm a stoner because I hang out with stoners piss me off.
People who assume any metal with screaming in it is "screamo". It's not. (Nu-metal, deathcore, death metal, thrash, metalcore... There's hundreds of genres in which there's screaming, but there's a difference between. A big one. Screamo is more like Senses Fail than Disturbed, Slayer, Beneath the Sky, All Shall Perish, Slipknot, Whitechapel, Winds of Plague... I argue this point with my sister all the time when she gets on my case for listening to screamo (usually Slipknot).)
People who assume doing metal vocals are easy. They're not. (To expand, I've found my throat bleeding from the screaming and growling.)
People who assume my girlfriend is fake because she lives in another state. (I'm in Texas now, and I've seen her twice. Yes, she's awesome. Sadly, I have to go back to Ass End Of Nowheresville, Illinois in about 5 days.)
People who assume I'm a delinquient because I say I listen to metal or am in a metal band. And people who assume I'm a fetus-eating satan-monster for the same reason (I got that from a discussion with the lead guitar of the band who drew me as a devil eating a fetus... don't ask, it's far too disturbing).
People who assume because I like metal, I'm officially unable to step inside a church without bursting into flames.
And the list goes on...