I try not to but it's bound to happen sometimes.
Normally when I get mad it has nothing to do with the core aspects of gameplay like getting out DMR'd by a spammer, having someone fire the pistol at you with machine-gun speed and somehow get a headshot, etc. Long ago I accepted the fact that this game rewards luck far more than it rewards skill, and it just doesn't bother me that much any more. Not much use raging over things that have always been part of the game and most likely won't change.
I usually play alone or with one other friend, and the thing that normally draws the most rage from me is my random teammates. My favorite teammate screw-ups are:
-When a teammate betrays me seven times in a row and I don't get a boot option (it's happened, I have the game recorded somewhere).
-When a teammate is pushing me forward in an enclosed space when I'm clearly trying to back up to avoid grenades or some other danger (happens like clockwork in any Invasion game).
-When a teammate just stands there watching me die instead of helping me fight a group of enemies.
-When a teammate betrays me and the game gives the kill to the enemy player that most recently shot me instead of giving my teammate a betrayal (I realize the game is designed to do this; I'll never understand why).
-When a teammate is betraying me for a power weapon that they most likely are not even capable of using effectively, which is why I picked it up in the first place.
Although I have to say, my all-time favorite thing to rage at that has been consistent across most Halo games is when the game goes to black screen just as I'm about to kill somebody.