Revealing mistake: When Sonny is punching Carlo under the spraying fire hydrant, he misses an audible punch by at least six inches.
Continuity mistake: When Sonny is shot at the toll booth the front windshield is torn apart by bullets yet when the bodyguards arrive thirty seconds later, the windshield is intact.
Revealing mistake: At the Godfather's funeral, as Al Pacino stands up, you can see a reflection of his mother over on the right of the screen. It's clear in the widescreen video. The same happens when he sits down again.
Revealing mistake: When Moe Greene is shot through his right eye, the eyeglass is cracked, but has no hole where the bullet penetrated the glass.
Continuity mistake: After Michael shoots Sollozzo in the head, he turns to McCluskey and shoots him in the throat and then in the head. But in the long shot, before he shoots McCluskey in the head, you already can see the hole in the forehead.
Continuity mistake: When they're all having a meal in Sicily, watch behind Appollonia. Two people, one in red, one in white, walk past in the same direction twice - once in a wide shot, and once in a close-up of Appollonia.
Continuity mistake: When Michael and Kay are eating lasagna during the wedding, Kay is smoking a cigarette. But depending on the shot, that cigarette disappears from her hand, then reappears.
Factual error: In the scene where all the heads of families meet shortly after Sonny's death, there is an exterior shot of a large grey-brick building with an American flag hanging from it. If you look carefully, the flag is a current flag with 50 stars, with alternating rows of indented stars. But, the scene is takes place circa late 1940's when there were only 48 states and all the rows of stars were even. (By the way, the flag inside the meeting room is the correct one.)
Continuity mistake: When Tom Hagen is meeting with Jack Woltz, the server fills Tom's wine glass twice within a split second.
Continuity mistake: When Diane Keaton and Al Pacino have dinner, he gets up, puts on his coat and sits down again. Now watch Diane Keaton's glass: When the camera's on Al Pacino she's holding the stem, but when the camera's on her she's holding the body of the glass. Another mistake is that at the end of the scene the amount of wine in her glass suddenly doubles.
Factual error: The Cadillac Miller and Cadillac Meteor flower cars at Vito Corleone's funeral are an anachronistic impossibility, given that he died some time in the summer of 1955. With two exceptions (one from 1951 and one from 1955), the Cadillacs serving as flower cars are 1956 models. Although a 1956 Cadillac Fleetwood 75 limousine also makes an appearance, it's plausible that the funeral could be taking place at a time when the first 1956 Fleetwood 75's were released. Miller and Meteor, however, did not begin converting 1956 Cadillacs into flower cars until 1956 proper.
Other mistake: After Sonny is shot on the causeway, there are a very large number of bullet exit holes visible on the sides of the tollbooths, which could only have been made if the gunmen shot through the walls (not the windows) of the buildings from the inside. This is common in movies. In real life, bullet entry holes in wood appear as small black dots, not the more dramatic-looking splintered gashes made on the other side of the wood as the bullet exits.
Continuity mistake: When Marlon Brando is gunned down at the fruit stand, he drops the bag of fruit he just bought. However, when he's collapsing against the car, the bag of fruit is now on the hood of the car.
Continuity mistake: When Don Vito Corleone gets shot, the camera angle from the side of the car shows Fredo bent over trying to pick up the gun he dropped, but when we see him from the camera angle from above, he's just walking to the front of the car. When the angle cuts back to the front of the car, Fredo is once again bent over trying to pick up the gun.