Here's one I've heard oft repeated in my circles - though I wasn't in the game at the time.
Twas fantasy hero - with it's quirky ruleset.
It seems as though this player had just joined a medium to high level group, and was forced to generate a new character at first level.
So she did - a trollish berserker with an axe nearly as big as he was.
During the first gaming session, they disturb a great dragon, and flee to a nearby castle built into a mountain. The dragon chases them all the way. The troll, moving slower than the rest, is naturally the last one in through the gate and portcullis.
He turns and sees the dragon shove his head into the portcullis and prepare to breath on the party. With a mighty war cry, the troll swings hist great axe into the creatures head.
roll: *crit*
roll: *crit*
roll for crit damage: *crit* 4x crit damage and instant death.
The first level troll sunk his axe as big as he was into the brainpan of the dragon that the high level party ran from, killing it instantly.
On the flip side...
I was gming an early 3.0 edition dnd game about the same time as the second revision of the PHB. We were still very much in a 2e frame of mind, and I hadn't yet let go of my crit tables.
A group of 10 or so bandits in a great plains like area ambush a 2nd and 3rd lvl party on the road with 2 pikes flanked by 2 crossbowmen in front and back, with bowmen to the sides.
The party refuses to yield up hard earned money, and attacks.
At one point during the relatively brief fight, one crossbowman has succeeded in reloading his crossbow, but tried to shoot it in the same round, with a fighter charging him on horseback... apparently this rattled him quite a bit.
xbowman: roll to hit: *1*, roll to confirm fumble *1*, roll for severity (my translation to my crit charts) *1*.
Rolled percentile, crit fumble table tells me "Throw weapon at opponent, roll for attack", that came out 1,1,1... Reconsult percentile "Attack nearest comrade" ... quick look at map shows 2 comrades, both 10 feet apart from this guy, both about the same amount of 'in the line of fire' ::shrug:: roll to hit one and see if attack lands... *1*... Hmm, roll for severity again: *20*... percentile and consult table again...
To make a long story short, after about 7 or 8 rolls, the xbowman tried to shoot the charging fighter, ended up trying to throw his xbow at him instead, fumbled that as well, tossing the crossbow itself up into the air. It came down on one comrade, whose reaction was severe enough in knocking it away from him that it discharged and shot the other comrade.
Much, much laughter from the players.