Oh this is probably already mentioned but right now it is something that is on the top of my irk list. People not playing to character. They will list out in their characters psychology on a cs that they act one way, then in rp everything posted for them comes out contradictory to that characters set up. A timid character who is quiet suddenly in every post is brave and mouthy. A character that always is supposed to think things through is always jumping in and never considering any angle. Or even worse, people who say their character suddenly know things their history and set have no mention of. A priest who has never seen a weapon much less battle suddenly is a sword master. A student in high school suddenly has the medical knowledge of the head cardiothoracics at the Mayo Clinic. And the worst for me is an Rper using their OOC knowledge and attitude to post for their character, combining both the above and then some to where you are left face palming mumbling "stay in fucking character!!!"