1. To make your OC seem like the most awesome, powerful, McBadass in the entire universe, expecting that every other player bows down to said character's awesomeness, and then kick up a hissy-fit when they call you out for it.
I encountered a player like that once; he was playing your typical edgy teenage mage character, but seemed to be of the impression that his OC was the most awesome thing in existence, given the way he wrote on the OOC thread. When I pointed out that he was engaging in meta-gaming (in that case, he wrote that his character had escaped from a massive courthouse in mere seconds despite having no prior knowledge of the place), he threw a tantrum and told me that I should "stop nagging and just RP".
2. Maybe this is just a pet peeve of mine, but I can't stand it when one uses ethnic personal names without first researching how they work.
I've seen more than a lifetime's supply of "Japanese" characters with names that either don't sound remotely Japanese, or are made of actual Japanese words/names that are misused to the point it's infuriating. Some I've seen before are "Akuma Midori" (just WTF), "Hana Usage" ("Flower rabbit"? Seriously?), "Reiko Ieyaru" (I don't even know where that surname comes from), and best of all, "Yokoyama Brooks" (when your name is made of two surnames).
I've also seen lots of Russian and Slavic characters whose "surnames" are actually their patronymics. Not cool at all.