The worst time this ever happened to me was when a black dragon (that we could not beat at our level) was going to let us go free after a literal hour of roleplay negotiations (basically, we annoyed the dragon to the point where he just wanted us to get the fuck out) but the dumb shit Ranger decided. unilaterally, that the party needed the experience because we had two new members, so we TOTALLY had to make a suicide attack, and rather than kick it off with Hunter's Mark and some bow-work, he decided to throw Firebolt as a "surprise attack" and beg for advantage, despite the fact that the dragon was watching us the whole time.

The DM ended up making the dragon just fucking leave after the party burned most of their abilities/spells, because he knew the Ranger would piss and moan if his character died. The Ranger's crestfallen face after the "You didn't kill it, so you don't get XP" announcement from the DM was my only enjoyment that night.