I remembered another scene stealing moment from Annie Hall: Marshall McLuhan.
Of course, McLuhan is not a professional actor (as shown by the fact that he flubs his line: “You mean my whole fallacy is wrong.”), but I don’t think that should disqualify this example. He still completely steals the scene, and his mistake barely registers the first time you see it, because his appearance is so unexpected.
I just realized that that’s one uninterrupted shot of almost three minutes, with intertwining dialog handled perfectly by the main actors. Little wonder Woody didn’t want to do another take.