Someone on Facebook kept trying to force one of my characters who didn't have interest in romance to become instant mates with theirs. I'm 100% fine with people jokingly (heck even seriously) shipping my characters with each other, but they were really just forcing them together and wouldn't accept no for an answer. I just began ignoring their posts.
Usually I'm fine with people making minor mistakes while drawing one of my characters, suggesting something OOC for my character to do (since USUALLY if I say they wouldn't do that, the other person understands and respects that), etc. but when people like the person above keep trying to force me to make my character do something they wouldn't ever do, that's where I draw the line.
Other than that, a lot of people tend to perceive some of my characters differently than they actually are and then force that character trait upon them, even if I tell them that they aren't like that, which bugs me a lot (misunderstandings are totally okay, but its not okay when the other people absolutely REFUSES to believe otherwise).