Continuity: During the first poker game, Vinnie is dealt a hand and then he rifles through them in full view of the camera (and the other players). The cards are easily visible as two Queens, a Ten, a Nine, and a Four. At the draw, he asks for no cards, and then he wins the hand, announcing he has a Straight.
Factual errors: The copyright date is shown as MCMXLVII (1947) instead of MCMLXVII (1967) as the copyright year for the film during the opening credits.
Continuity: When Oscar is at the ballpark, the phone rings and he puts his hot dog down. When the man answers the phone, Oscar is typing and the hot dog disappears. When we cut back to Oscar, he is eating it again.
Continuity: In the opening scenes, when Felix is walking into the go-go dancer club, a woman with long brown hair is leaving. After Felix enters, she is dancing on stage.
Errors made by characters (possibly deliberate errors by the filmmakers): In the restaurant, when Felix is doing his nasal honking, Walter Matthau looks like he is trying to contain some laughter before he says, "Stop that, will you! What are you doing?"
Continuity: In Oscar's apt., there is a wall lamp on either side of the front door. Each lamp holds two bulbs, but one of them is missing a bulb. This missing bulb jumps back and forth between the two lamps throughout the opening scenes.
Continuity: In the restaurant, after the waitress leaves, while Oscar is laughing, Felix picks up his handkerchief and starts folding it. Then in the next shot, he picks it up and starts folding it again.
Audio/visual unsynchronized: Audio/Visual mismatch between the shots where Felix says "How's she gonna meet somebody now at her age with two kids, and where?"
Continuity: The spaghetti/linguine splatter pattern changes at least three times.