This might have been said already... but I get real irked when someone dresses up as the same character as someone else (or you) and acts all better than the other for some reason.
Another thing that irks me is when a group of people (or two) randomly bust out into a fight for attention. Besides the fact that's dangerious... it hogs attention and makes moving around the halls impossable.
Before Crisis and In Game Rufus Shinra~ Final Fantasy VII
Mello~ Death Note
Seras (yellow)~ Hellsing
Alexia Ashford~ Resident Evil Code Veronica
Reno~ Final Fantasy VII