In one of my pathfinder games, we had a cleric who insisted upon using a crossbow, even though he had almost no skill points in using it.
At one point, he managed to KO two of the four active party members with missed shots. After doing this, tries again.
He rolls a 1, and hits the already unconscious rogue.
And then rolls a nat 20 for damage. And confirms the crit.
Luckily, the rogue had some fancy feats which let him not be instantly killed.
Our ranger (who luckily has the precise shot feat, so no killing allies) finishes off the remaining bad guys while the cleric stabilizes the downed members.

We took the crossbow.

The game was EON and my character was actually dead.

A templar he died fighting an undead dragon.
It went like this see with a mighty thrust he slays the dragon, but being a poor sport the dragon attempts one last breath attack, however it misfires so all that comes out is ether (acid) it splashes my templar... on the crotch, scurrying to get of my slowly sizzling armor I am just too slow.
The ether bites into my skin and I die from it's poisonous qualities.

This was after 2.5 years of playing two or three sessions a week.

My friends one a templar and the other a priest of the same order managed to fetch a shaman (soul magicians) and stuffed my soul back into my body (after giving said body a quick heal).
So after all I actually died, but alas the gods did not deem my work to be done.

I was playing an encounters session one time at the local game store. Being a new player, I didn't understand how temp hp worked, so I ended up having something like 30+ temp hp. I'd mentioned it a few times, but no one seemed to care (I was playing a Warlock). So anyways, we're going along and the next thing we all know, there were enemies everywhere. The party had gotten split up, so half of us were in some type of bar and the other half were outside. Those of us outside are trying to fight our way in (I think it was me, a barbarian, and something else) but by the time we were close those inside had been slaughtered and we'd lost the third man. Then the barbarian went down. I barely managed to escape, having gone from close to 50hp, to 12ish. It was not a day of glory.

Dwarf sorcerer solo campaign. Dodge bonus VS giants.
Vengeance by nasty, nasty spells to, as I recall, boil the frost giant's blood, set it (the BLOOD) on fire, and inject acid into its bloodstream.
I love the sound of screaming. 'Course, his pet ankheg nearly got me, and I had to flee.

3e GURPS campaign back. Medieval/fantasy setting with only secret magic (or no magic) e.g. no mages, healing potions, etc.

My archer takes a crossbow bolt to the torso. I had taken no damage before this, but he rolled high and took me from full HP (11) in this case to exactly negative HT (-22), and I failed my death check.

Except GURPS has "blow through" rules for damage, since crossbow bolts and arrows aren't really known for magically ricocheting around your torso. We realized the next day that I should have taken damage equal to my full HP score before blow through kicked in.

Thus it was ruled I just passed out, and so that instead of dying and avenging me, my unconcious character was rescued. ;)

Had a druid in a 3.5 campaign... party got attacked by wave after wave of rogues, took lethal damage. mourned my death for a moment, then managed to fast talk my dm into agreeing that I'd shift back to human ad upon doing so would get the healing you get from shifting back and that brought me back to life... I dunno if that's how it's supposed to work, but it kept me alive so I'll take it :D

There's a curse in Forgotten Realms that makes you come back from the dead as an undead warrior, nothing but adding the undead template really, I've used it on purpose to cheese my way out of death on purpose, but the first time it came up was pure chance.

My rogue got a curse we didn't find out what it was, it was a plot point for awhile.

Anyway, GM has a rule about ID'ing things after death as a freebie closure thing.

Rolled a die, compared it to the table.

Stare at the results for a minute or two.

Reroll die to check for it being weighted.

My rogue got back up.

A 4th Ed Warlock had an unfortunate start - being part of a first game TPK. I had made him a 'lock of all trades, sort of, and with his abilities he teleported all around the battlefield, taking a sword swipe here, patching up a downed member of the team there. Ultimately though, of five characters, two survived due to heal checks made on the way to combat with the final enemy.

Being kinda generous with information, the GM advised that my hit had reduced the lizardman to exactly 1hp, but my damage roll simply had not been enough to kill it. The Warlock was simply not going to survive the next attack. But he had a trick - a curse that he placed on the lizardman, so that if it dealt him damage, it would take 1 damage in return.

The lizardman felled the Warlock. The Warlock was not able to stabilize, and died while the two stabilized team mates lay unconscious nearby, unable to help, and the other two had been fully slain already.

But that curse did its one damage.

Not exactly killed persay but knocked out

I was playing 5th ed dnd and during a random encounter we ran into a bunch wolves(The DM rolled max for numbers) and each attack that hit would cause a strength saving throw with DC 11 or be knocked and me being a dex fighter had a horrible strength Stat so I got knocked prone multiple times and had taken more hits and had less movement because of this so by the time I had dealt with all the wolves on my side I was badly hurt but the wolves had split up to attack from the back and the front and the other wolves were after the squishy casters.So I went into the fray again and got knocked out.

After the fight ended I remembered that I had both proficiency with strength saving throws and a magic item that gave +1 to all saving throws turning my -1 strength save to +2 which I probably would have made the save more meaning less atacks.

*knocked down