Tuna
Details of an actual event while playing a D&D one shot:
DM: The cult leader stands there, surrounded by his guards. You are first in the initiative what would you like to do?
Me (The party's druid who is just looking to fuck around): I want to cast polymorph.
DM: Ha! *rolls dice* I doubt that it's going to hit---
He goes silent for a full five minutes. Just staring at his dice.
Me: it hit, didn't it?
DM:................
DM: what does he turn into?
Me: a tuna.
DM: What
Me: You heard me.
The BBEG does in fact turn into a tuna. He proceeds to flop around as we defeated the rest of his guards. The moment he turned back my friend's minotaur charged at and speared him with his horns, effectively killing him. The villain never got to fire a single shot at us. I do believe we gave our DM a headache that night.