After the wedding, the limo window opens, and a hand, presumably Jack Lemmon's, drops a dead fish onto the pavement. Walter Matthau is standing on the sidewalk, enjoying the moment. Jack Lemmon comes up quietly alongside, and asks him, "Who left?" Matthau does a double- take, and shakes his head, laughing.
Burgess Meredith, as Grandpa, is shown acting out different versions of one scene (A245/84B or A246/84B). Presumably the nature of the scene meant that the choice of which version to use had to be made later.
One outtake has Jack Lemmon flub his lines, and he continues, in the same tone of voice, babbling gibberish. Matthau answers him in the same "language." Their timing and intonation are precisely as they'd be if they weren't jabbering nonsense to one another.