Factual error: Michael had his meeting withe Solozzo around Christmas of 1945. On the way to the meeting, the car made a U-turn on the George Washington Bridge (GWB) to throw off anybody who might be following. At the time, the GWB had four traffic lanes, two in either direction. The median between the two set of lanes was about 20 feet of steel mesh, making such a maneuver impossible. The median was filled in to add two additional traffic lanes in 1946. At that time, the u-turn would have been possible.
Continuity mistake: At the restaurant, Michael, Sollozzo, and Captain McCluskey have just sat down at the table and the waiter comes over to take their order and pour wine. McCluskey asks Sollozzo what's good to order, and he recommends the veal. Before the waiter leaves the table, McCluskey is suddenly eating his dinner, even though they just arrived. By the way he is using his knife and fork, it is obvious that he is eating an entree, and not a salad or an appetizer.
Other mistake: As the camera pans in on Woltz's mansion right before the horse's head scene, his bedroom is on the second floor. However, the windows in his bedroom show a courtyard in the background. The horse's head scene was filmed in a dining-room-turned-bedroom at a Guggenheim mansion on Long Island. The Guggenheim dining room is located on the first floor, as shown in a February 2022 episode of Antiques Roadshow.
Factual error: In the establishing shot of the movie producer's mansion, a car in the driveway is suppose to be a 1937 Cord, but actually it is a replica produced between 1968 and 1972. Real Cord cars had bug eye headlights that cranked out of the fenders. The replicas had fixed glass covered headlights in the fenders.
Revealing mistake: When Tom is visiting with Waltz, it is a body double and not Robert Duvall walking outside on the way to see the horse.
Other mistake: When Michael arrives in Las Vegas at the Hotel Continental and exits the car two men are visible through the window inside the hotel who are dressed in 1972 contemporary attire, despite this scene being set in the 50s. The man on the left in a blue jacket looks as though he could be David Geffen. (02:19:38)
Visible crew/equipment: During the meeting between the Corleone family and Sollozzo to discuss drug dealing you can see a boom microphone reflected in the glass window when Don Corleone stands up to pour Sollozzo a drink, it can be seen moving around. A little later in the same scene when Don rejects Sollozzo's offer you can see a crew member reflected in the same set of window's moving around (above Sonny's head). (00:37:20 - 00:38:44)
Other mistake: While Michael is talking with the Sicilian father, requesting to meet his daughter, the camera jostles twice, between the close shots of both men. (01:40:00 - 01:46:00)
Continuity mistake: When Woltz shows Tom Hagen his horse, Khartoum in the stables, the horse has a prominent white star on his head. The horse's head in Woltz's bed later that night has no such star, the head is all black.
Continuity mistake: When Tom is walking down the alley between buildings at Woltz Studios before he meets Mr. Woltz, the bag he is carrying looks like a travel bag and is almost equilateral triangle shaped. When he meets Mr, Woltz, his bag is a much thinner (isosceles triangle) shaped briefcase. In almost every shot the hand carrying the "bag" is switched. (00:27:45)
Continuity mistake: At the end of the shot where Al Neri shoots Emilio Barzini's bodyguards, the one on the right is still in the process of falling and hasn't hit the ground yet before it cuts, but in the next shot, he's already on the ground and not moving.
Other mistake: When Al Neri shoots Emilio Barzini's bodyguards, they're both only shot in the torso, but when they're on the ground, there's only blood coming out from under their heads.
Continuity mistake: When they leave to kill Pauli it's light outside, when they are driving and Clemenza says that they to get more mattresses it's dark outside, when they stop to kill Pauli it's light again.
Audio problem: When Vito Corleone is brought back from the hospital and he is upstairs in his bedroom with his other family members, after his grandson Frank gives him his gift, the baby who was just crying has his mouth completely closed while the audio has him still crying as they leave the room. (01:33:00)
Chosen answer: The mattress montage music is called "This Loneliness." It is written and performed by F.F.C's father.