When getting ready for their dinner with the ladies, Oscar jokingly asks, "You think Mozart goes good with meatloaf?" Walter Matthau was a passionate Wolfgang Amadeus Mozart enthusiast, and had extensive knowledge of his work.

The two great friends, Jack Lemmon and Walter Matthau, are paired in a movie for the second of ten times.

Walter Matthau, who played Oscar in both the original Broadway play and the movie, asked the play's author, Neil Simon, if he could play Felix instead. This was because Matthau thought Oscar's personality was too similar to his own and the role would be too easy; whereas playing the persnickety Felix would be a real acting challenge. Simon replied, "Walter, go and be an actor in somebody else's play. Please be Oscar in mine." Matthau finally agreed to it.

Monica Evans and Carole Shelley reprise their roles in Disney's The Aristocats (1970) as a pair of English geese on a walking tour of France.

According to former Paramount production chief Robert Evans in his memoir "The Kid Stays in the Picture", producer Howard Koch originally wanted to use the Broadway cast, Walter Matthau (Oscar) and Art Carney (Felix) in the movie. Evans wanted Jack Lemmon for Felix. Evans also wanted Billy Wilder, who directed Lemmon and Matthau in The Fortune Cookie (1966), as writer-director. The cost for the Lemmon-Matthau-Wilder package was $3 million plus 50% of the profits. Paramount owner Charlie Bluhdorn balked at the demands and personally took over negotiations. Wilder eventually dropped out. Lemmon was signed for $1 million against 10% of the gross and Matthau got a straight salary of $300,000.

Continuity

When bowling, Felix gets a strike then says, "When you're right, you're right." Then goes to mark the score manually. He looks up and counts the pins left standing. But, he got a strike.

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.

After Oscar throws Felix out of the apartment, the guys are out searching for Felix in Murray's police car. When Vinnie comes out of Felix's old apartment, they are seen driving in police car #578. In the next scene, they are parked and two other officers pull up in car #578, and the guys pull off in car #527. In the next scene, they are back in #578 again. When they're heading towards the park they are in car 956.

The apartment's kitchen window is closed when Oscar and Felix enter it together the first time. After Oscar asks Felix to get him a pot from under the sink, the window is open, which is later shut by Oscar.

At 0:06:25 uncredited Go-Go Dancer is seen leaving, as Felix is entering. The Metropole Cafe. At 0:08:11 the same Go-Go Dancer is shown in close-up in red two piece, dancing on the Metropole stage.



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.

When Murray the cop is dropped off at the poker game by an NYPD Radio car, the car's side says "PCT. 231," meaning the 231st Precinct, which does not exist, nor do the precinct numbers get anywhere near that high. The highest numbered precinct in the NYPD (There are seventy-six Precincts today- seventy-NINE in 1968)) is in the Tottenville section of Staten Island, and that's the 123rd Precinct.

One cannot smell the difference between spaghetti and linguine.

Nobody orders four pounds of hamburger to make a meatloaf for four people. Also, Felix makes a big deal about the meatloaf being ready at a particular time. Meatloaf is actually one of the most tolerant foods about cooking times and holding until ready to eat.



Incorrectly regarded as goofs

Felix plays a pinball machine in one scene. Pinball was illegal in New York City from 1942 until 1976, but pinball machines did exist. The illegal pinball machine is likely intended to demonstrate the seediness of the pool hall.



Revealing mistakes

When Felix and Oscar are entering the apartment (while mad at each other), Oscar slams the door in Felix's face. When the door slams the whole wall shakes.

When Felix first arrives at Oscar's apartment he asks if anyone has called for him. After Oscar tells him no one has called, the guys continue to bet on the poker game. One of them says, "I raise a dollar" and the sound of a poker chip spinning on a hard surface can be heard. The table they are playing poker on is covered in felt, which would not make that noise.

While searching for Oscar in Murray's patrol car, the driver's-side vent window frame appears in long shots, and disappears in close-up shots.

When Felix throws the cup at the wall it makes an obvious hollow sound of cup against plywood and not the wall of a room.



Miscellaneous

In Oscar's apartment, Felix looks out the window and asks "How many floors is this, 12?" But since he comes to the apartment using the elevator (and therefore pressing the floor button) he would already know what floor it's on. Perhaps it was a rhetorical question.

As Felix and Oscar walk past the supermarket, Mackerel is misspelled on the marquee as Mackeral.

In the opening credits, the date is shown as "MCMXLVIII" which is 1948. It should be "MCMLXVIII", which is the correct date of 1968.



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?"