There is one type of person which really pisses me off. Actually there's more than one, but only one I'll talk about here and now.
It's the kind of person who will wander along while you're obviously in the middle of a conversation with somebody else, and just start talking to you and expect you to break off your other conversation, drop everything, and listen. I mean, it's glaringly obvious that you're talking/listening to somebody else. But this type of person just doesn't see or doesn't care. They just butt right in. It really really gives me the utter shits.