It was in 3e. The party raided an encampment of thugs led by a foreign blackguard who had been infiltrating local criminal elements in order to weaken civil society in preparation for an invasion. The party paladin had a serious hate-on for this blackguard guy. Not only had the blackguard pulled the wool over the PCs eyes once or twice, but, you know, paladin, blackguard, seemed a pretty obvious rivalry, and the player bought right into it.
The fight wasn't a tough one, in theory. The thugs were wussy, while the blackguard was a PC one level below the party. So a little difficult, but not very. In theory.
The paladin charges the blackguard in the first round of combat. Probably a good move, the blackguard had a greataxe and could do some hefty damage, and the paladin had the most hit points and armor. The rest of the party moved to shred the thugs while the paladin held the blackguard back, with the intention of joining up with him and gang-beating this blackguard in a round or so.
The paladin misses his charge attack.
The blackguard attacks back. He has two attacks per round. I make one a Smite Good, because why not, right? Its logical.
Critical hit. Second attack? Critical hit. Both confirm.
I curse rolling in front of the players. Curse it! Silently. The paladin's player is all quiet all of a sudden, and wants to see me roll damage. He's actually upset. This is TOTALLY not the way he wanted things to go.
I do math in my head pretty fast. Way faster than him, apparently. Because I already know that the paladin is a dead man. 3x regular damage? With extra smite damage? He's screwed. Total damage is going to be 3d12+21 + 3d12+12. Minimum damage here is 39. He's a level 7 PC with average hit points, meaning low 50s.
He dies in the first round of battle, against the first foe who attacked him, on the first attack. In his climactic showdown with his arch rival.
Poor guy almost cried.