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.
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.
After all the titles, Walter Matheau in the bathtub proclaims. "If I knew there would be a nude scene in this film, I'd have asked for an extra million"
Another outtake has Walter Matthau saying... Hello, I'm Ronald Reagan, I used to be president of the United States. I live across the street. I was also in the movies for a while - I was a lousy actor
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.
Take 2: Looks like he's going to enter the holy of holies .. coitus uninterruptus.
Take 5: Look's like Chuck's slipping her the old salami.
Take 7: Looks like Chuck's going to put the hotdog in the bun.
Take 10: Look's like Chuck's taking the skin boat to tuna town.
Take 13: Looks like Chuck's a tomcat on the prowl.. Meeeeoowwwwww
Take 13 (also): Looks like Chuck's taking the old log to the beaver
Take 14: Look's like Chuck's gonna bury his boner.
Take 16: Look's like Chuck's taking a ride on the wild baloney pony.
Grandpa's lines were as follows. N.B. He's just seen Chuck enter the home of Ariel.