Bigotry. Blatant and to a lesser degree inadvertent hypocrisy. Sexism. Prejudice. Stupidity, ignorance notwithstanding. Arrogance. Illiteracy. Double standards. Mob mentality. Law system instead of Justice system, where the form and letter of the law gets in the way and at times outright prevents justice or even creates grossly unjust situations, actions, and results. Abuse. And last but not least, getting my hair caught in my mouth and gagging on it!
I find this one funny, I've noticed it too. People tend to spell how a word sounds like "Greif, Cheif, Theif" instead of following "I before E, except after C, or when sounding like /ay/ as in neighbor or weigh." Edit: Should probably also add that not everyone's first language is English
Hackers. In MineCraft, this is the only thing that truly made me RAAAAAAAGEEE!!! Not long ago, when just having fun on a Factions PvP Server, the Owner himself killed me with hacks. Are you kidding me? Why would an OWNER hack?
... Hackers in MC - People who use Cheats such as FlyMod, Nodus, Aimbot etc. Hackers in Real life - People who, well.... Hack stuff.
New thing that annoys me: Teams that never have a Medic in TF2. It's downright ridiculous at times, since most games I end up joining don't have a Medic on either team. I don't like playing as the Medic, but I'm always the first, and probably the only one to change to it. It get's excruciating after a while, and very pathetic when my team keeps winning, just because my team has a Medic. Really...
That too, there was this one Scout who kept mashing Medic button, but didn't need to be healed. After going to him 5 times, I ignored him, and let him die. He was SUCH a prick in that game.
Just saw another. When someone spells it "chocking" instead of "choking" I just feel like punching them in the face.