Revealing mistake: When Walther Matthau is making sandwiches for his poker pals (during the movie's first poker game), Matthau drops a slice of bread on the kitchen floor. If you look at his hand, you'll see that he flicks his thumb to make the slice of bread fall.
The Odd Couple (1968)
1 revealing mistake - chronological order
Directed by: Gene Saks
Starring: Walter Matthau, Jack Lemmon, John Fiedler, Herb Edelman
Continuity mistake: In the opening sequence, Felix (Jack Lemmon) goes into the Metropole Cafe. One of the people he passes as he enters is a young woman with a thin face and very distinctive nose and eyes. He goes in, has a drink, hurts his neck, and leaves. As he is leaving, the camera pans the band and go-go dancers. One of the dancers is the young woman who had just left as he was entering.
Vinnie: I thought he looked edgy the last couple of weeks. Don't you think he looked edgy?
Speed: No, as a matter of fact, I thought you looked edgy.