Wall of Stone.
My player (a dwarf druid) wanted to cast the wall of stone in the air and have the whole stone wall fall down on top of a beholder for ungodly amounts of damage.
Like an idiot, i allowed it.
The campaign is over now, and this has always bothered me because i never looked the spell up for myself, just kinda trusting the player.
The spell shouldn't have worked that way. The spell clearly states that it has to be attached to something or "merge with and be solidly supported by existing stone." So i cannot even be an unfinished bridge ready to fall down or anything either.
I thought he found a chink the game. i worried that he would spam that spell for the rest of the session. Fortunately, he dropped out due to unrelated personal reasons.
The point is: I trusted him to know what the hell he is doing.
And he didn't.
The beholder died.