I had an indirect one, of sorts. I helped a friend who would be playing a campaign with one of our usual DMs who usually makes outrageous but fun settings, but we convinced him to tone it down for one campaign to regular vanilla stuff. He agreed on the condition players could only use PHB stuff.

Level 6 Fighter with a Greatsword, very bread-n-butter Charge + Power Attack. But I threw in Cleave and Great Cleave too because "why not?"

Fast forward, the party was ambushed by some gnoll scouts. As the fighter dismounted the carriage, he was rushed from all sides by gnolls before his turn came up. He attacked, killed one, Cleaved, killed another, player declares the next attack. DM says he can't cleave agains unless he has great cleave, the player replies that he does and shows him his character sheet. The DM grabs it and goes pale and makes a pouty face before saying "I didn't know..." and giving back the character sheet to the player. 5 brave gnolls died in a single round because the DM forgot that Great cleave's requirements are hilariously low.

One that was my own was a Fighter/Rogue going full Shadow of Mordor on the smug Wizard trying to convince him to kill someone on his behalf in exchange for some critical information the party needed. Fighter sat in front of the wizard's desk, chatted amicably with him, even offered to play chess with him and sell some of his loot in exchange for the info. When the Wizard got fed up and demanded an answer("Will you go kill the guy, yes or no?"), Fighter goes: "I think..."

I declared a surprise round, tumbled on top of the desk and sneak attacked the wizard for 20 damage. Roll initiative, fighter goes first. Full attack, the wizard only realized what happened when his head toppled to the floor, shock on his face, his body still sitting on his chair and the fighter standing next to where his head had landed. "You're not so smart if you think I'll take whatever deal YOU offer me."

Fighter is a Chaotic Good Cowboy Cop who lives ON THE EDGE and doesn't make deals with terrorists.