As well as several which have been already mentioned, posts which annoy me include:
Obviously fake typos in thread titles e.g. "AIBU to not wank [want] this?" which the OP will then claim is an autocorrect, oh silly me teehee.
Posters who are third or fourth to reply and ask a question clearly answered in the OP if they only bothered to read it. E.g. OP: "My son is 7 and wants to play football and learn violin, is this too much after-school activity?" Poster #3: "well it depends on his age OP, how old is he?"
And my ultimate pet hate: posters who reply in a longish thread simply saying "100% what X said" / "OP you should absolutely do what X said" / "X has summed it up perfectly" but they don't quote X so you have to scroll back hoping X's post will be just a few posts further up from their comment or at least on the same page, but it never is!