Quirrell's Troll is Already Knocked Out

“We’ve had Sprout’s, that was the Devil’s Snare; Flitwick must’ve put charms on the keys; McGonagall transfigured the chessmen to make them alive; that leaves Quirrell’s spell, and Snape’s.”

They had reached another door.

“All right?” Harry whispered.

“Go on.”

Harry pushed it open.

A disgusting smell filled their nostrils, making both of them pull their robes up over their noses. Eyes watering, they saw, flat on the floor in front of them, a troll even larger than the one they had tackled, out cold with a bloody lump on its head.

“I’m glad we didn’t have to fight that one,” Harry whispered as they stepped carefully over one of its massive legs. “Come on, I can’t breathe.”