Continuity mistake: When Michael enters the room where he sees Kay looking nostalgically at the old photos, he is carrying and offers to her pieces of cake from a cake that hasn't even been cut yet. They are the exact size and dimension of the cake slices that will betaken from the celebration cake which is not cut until several scenes later.
Continuity mistake: When Michael, Anthony and Mary are walking along a wall, Anthony is holding a sheet of paper which he is studying at the beginning of the first shot in the scene. The paper is the picture of Michael in his limousine that Anthony drew as a child (and is seen in Godfather Part II). However, Anthony isn't given this picture until later in the film. Notice that they're wearing the same clothes in both scenes. Clearly a much longer scene was cut into two parts, and the second half is seen first. (01:31:35 - 01:51:55)
Continuity mistake: When Joey Zasa comes to Michael to complain about Vincent, Michael asks Al Neri to get him. Al leaves and we hear the door close behind him. Then Vincent comes, accompanied by his aunt, but Al is nowhere in the picture. But when Vincent does a Mike Tyson on Zasa's ear both Vincent's thug Squigilaro and Al come to separate the two opponents. (00:23:45 - 00:27:30)
Continuity mistake: When Calo assassinates Don Lucchesi, he grabs the Don's glasses and stabs him in the upper chest about three inches below the collar of his shirt, yet later, when Lucchesi is shown slumped in his chair and bleeding, the glasses are seen sticking out of his neck above the collar.
Continuity mistake: In the scene where the Corleone's are travelling to the house in Bagheria they are in 3 cars. When they arrive at the house they are being escorted by 2 motorbikes not there in the previous scene. (01:21:30)
Continuity mistake: During the scene that takes place at the party were Connie is singing the song about the instruments, Vincent Mancini and his mother can be seen arriving to find out their names have not been listed to attend. The shot switches back to Connie singing, and Vincent's mother can be seen at a table enjoying the song and festivities. It then cuts back to the entry hall where Vincent begins to shove people around, and his mom is still standing there looking upset that they have not been admitted to the affair.
Continuity mistake: At Michael's celebration there is a scene of a family photograph taken on the stairs of his home. When the photographer is taking the picture of the family, you can see that the camera he is using is a Sinar P1. But when the scene's angle is reversed, you see the front of the camera, and it is a completely different one - a Toyo-View G.
Continuity mistake: When Mary is hit at the end of the film and Kay screams, Vincent looks up and shouts "No!" and the scene cuts back to Michael holding Mary, as we hear Vincent continue his line ("Mary!") but then Vincent appears behind Michael and Mary, walking slowly up the steps, hair slicked back, gun in hand, mouth closed as his line is still being delivered, with no pause ("Mary! No!"). The next cut shows Vincent back at the bottom of the stairs near the priests, with his hair mussed up. And the poor editing continues - when the camera pans out following Michael's scream, we can see Vincent slowly walking up the steps behind Michael, and this is the same shot as before from a different angle.