Haha, great Halo 2 moment:
I'm running around on some map (can't remember the name-blue foggy one, with lots of walkways.) when I see my friend on the other team calmly sniping from a small alcove. Now, I'm not a bad Halo player, not by a long shot, but this guy is ungodly. Anyways, I run up behind him (no motion sensors) and jack him in the back. I grab his sniper, and run off to find a better spot. I'm going down a walkway when I randomly decide to turn around, just in time to dodge a melee from my friend. Scared out of my mind, I shoot him dead with the sniper from three feet away. Both kills happened within 30 seconds of each other. From upstairs, I hear him scream "What the ****? NO!"
I laughed for a long time about that.
Another situation, same game: Me and the rest of my team were all randomly crowded in a room (it was 4v4) all pointing towards one of the blue hoverlift thingies, when who else should come up (all by his lonesome) but the same guy. I had duel SMGs, and the other guys had a mix of plasma, pistols, and SMG's. The poor guy had a battlerifle. There was a split-second moment where we all just stared at each other, and then he exploded in a big mess of grenades, plasma, and armor piercing rounds.
Later on he told me that his only thought when he saw the four of us standing there was "oh jesus christ, **** me."