For me and my friends:
Black Ops: I threw a grenade, it hit a tiny pipe and it bounced back and hit me between the eyes and detonated.
Red Dead Redemption/Undead Nightmare: 1: Ran a horse off the bridge to Mexico because I didn't notice it was broken. 2: (For a friend) Was on his horse in Tall Trees, a wretch-er that wasn't supposed to be there spat on the horse, the horse fell down the mountainside, survived, then was eaten by 10 common undead. 3: (For a different friend) Started killing wolves with a knife while hunting, there were about 10, two left, they Killed him. 3: (For a friend mission fail type death) A woman's carriage was stolen, he almost got away, after fiiiinally catching up and bringing it back the woman was attacked by a cougar while she was running up to meet with him. There are more I just can't remember.
TF2: Got killed by a Heavy's taunt three times in a row then the Heavy stole the intelligence and won the match. There were more but moving on.
L4D/L4D2: Accidentally jumped out a window in Dead Center. (For a friend) Startled a witch, closed the door and ran about halfway through the level (backwards) and the witch finally caught up and diced him. There are more but moving on.
I'll likely remember more soon.