"Whoa -- hey!"
The party was undercover in a town that had been secretly taken over by a demon, and all the clergy and prominent citizens possessed. One of the characters was wandering around at night on a flying carpet with a ring that was a variation on a Wand of Wonder. He came across three guys who were levitating near the roof of a temple, trying to remove a body my character had stuffed down the chimney earlier in the evening (The demons had been turning people to wood and sacrificing them in temple fireplaces.).
He'd been wanting to do some testing on the ring, so he fired it at one of the guys, who disappeared. The others reacted, the PC ducked out of sight, and the guy reappeared in a different location and yelled "Whoa -- hey!" Stuff happened, the guy disappeared again, more stuff happened, he reappeared and yelled "Whoa -- hey!" Stuff, disappearance, stuff, reappearance and "Whoa -- hey!" And three more times.
The DM had noticed us quietly laughing after the exclamations -- and trying not to laugh before them -- but it was only as he said it the seventh time that he realized what he'd been doing. He sort of half laughed/half cried, but felt obligated to continue it for the rest of the encounter. Since then we've shouted it out for NPCs on his behalf, when it seemed appropriate and we could get it in before he said "But he doesn't say...'"