When people assume shit about your character, then get pissed when they find out they weren't right.

An example of this I run into a lot-

People assume your character is a top/Dom/seme/whatever, then when they find out their actually a bottom they get pissed and demand  that you change it because they don't think it suits your character and because they wanted to play the bottom.

Firstly, fuck you. Can't stress this enough but

Tiny shy characters can be tops

Huge, tough characters can be bottoms

And you don't have the right to tell anyone that they have to change their characters in their roleplay.