Well heres a few I can think of off the top of my head.
- That teenagers are all irresponsible, immature and irrespectable.
- That old people can't be idiotic, immature and completely irrespectable.
- That women can't have male friends.
- That men can't have female friends.
- That a man interested in dancing, art and fashion is auotmatically gay.
- That a woman interested in sport, automobiles and I dunno, carpentry? is automatically gay.
- That people such as myself who love death metal and wear alot of black clothing aren't as approachable and nice as anyone else.
- That video games and animation with alot of color and style can't be mature and as interesting and entertaining as anything aiming for realism.
- That anime doesn't come in many diverse forms and consists solely of soft porn, giant robots, and regular porn.
- That all furries are sick weirdos who dress in strange outfits and spend every day jerking off to disney cartoons.
- That anyone who enjoys "stupid" movies such as Transformers can't also enjoy and understand movies with depth, meaning, complexity and artistic merit.
I should also point out that I can see the humor in alot of these steriotypes and often find them as funny as anyone else, but eh the general genuine beliefs alot of people have that lead to these misunderstandings of so many things still really bugs me.