People assume I'm a Christian because I talk about God, have pictures of the saints around my home and am happy to talk to Christian people in the religious context of their understanding. I'm actually a Satanist.
People assume I'm heterosexual... until my boyfriend shows up.
People assume I'm a cisgender male (or if online, female). Sometimes that involves them saying nasty things about men (online), transgender, transsexual and other-gendered people in my presence. I then clarify that I am trans male, and I don't appreciate that sort of talk around me.
People assume they are talking to only one person when they talk to me. I'm multiple/a soulbonder, so there are actually a few people listening in.
People assume I'm an intolerant jerk. In reality, I am just... VERY vocal about ethics, gay rights, trans rights, the right to freedom of religion, help for people who are disabled, the right to medical treatment, etc. I try very hard to listen to all sides, and I try very hard to be civil, but the minute someone starts spewing homophobic bullcrap, I'm gone.
People assume that just because I'm "fat" that I constantly eat poorly and don't work out. Before I got sick, I used to exercise for two hours a day, and I was eating vegetarian as well as gluten-free. Aside from my main health issues, I have pretty good health; my cholesterol and blood pressure are good.
People assume that I am healthy. I walk with a cane frequently because of leg and hip pain; I have fibromyalgia; I have PTSD; I have an anxiety disorder, and I have chronic fatigue.
People assume I have money, just because I have the internet. The internet is one of my few luxuries. My family doesn't make a lot of money. We've never owned our own home, and we have rented the same dumpy place for nineteen years. We have only gotten one new car in the nearly 24 years I've been alive. We're not starving and dying on the street, or as bad off as some people, but I feel really down when people assume I have an iPod, a cellphone, a car, etc. I don't even have my own room, let alone an iWhatever. :|