Factual error: When an adult Vito returns to Sicily, and visits Don Ciccio to take his revenge, the sea can be seen over his shoulder. There is no sea anywhere near Corleone, Sicily.
Continuity mistake: When Senator Geary talks to Michael in his Lake Tahoe office and the camera is on Michael there's an older couple (the woman is wearing a fur jacket) walking by outside. The camera cuts to Geary for a while, and when it cuts back to Michael the same couple is still walking by. (00:16:15)
Factual error: After the attempted murder of Frank Pentagalli takes place, as the shootout in the street continues a Mark II Jaguar is parked across the street. Not in 1958.
Visible crew/equipment: In the flashback scene shortly after young Vito has killed Fanucci and then joins his family on the front steps, a boom microphone is visible above and to the right of the steps.
Continuity mistake: On the porch in one of the flashback scenes, young Vito is holding baby Michael. Behind them is a guy playing a Gibson-style flat-top guitar that hadn't been invented yet. (02:00:30)
Continuity mistake: When the talk between Senator Geary and Michael gets tense the water in Geary's glass doubles. (00:17:15)
Continuity mistake: When Vito's mother asks Don Ciccio to spare her son's life the Don's collar is folded narrow and close to his neck. When the camera cuts to a wide shot before he gets up the collar is wide and flat on his vest. (00:04:20)
Continuity mistake: When Michael tells Fredo that he doesn't want to see him again Fredo is lying shattered in his reclining chair with his arm hanging down. When the camera angle changes Fredo's arm is lying on the armrest. (00:24:45)
Other mistake: When Paolo is shot dead, there's blood dripping down the rocks he's lying on, but there shouldn't be. The rocks are under his shoulder, and he was shot in the back, and a little blood dripped out sideways. And there's no blood on his shirt where his body is touching the rocks.
Factual error: Corleone and Roth meet in Havana in Christmas 1958. The initial scene shows a panorama of a full street with people and American cars passing by. Among the cars we see a Jaguar Mk.2 in the opposite lane driving away. New Years day 1959 Batista flees the country and Fidel Castro takes over. The Jaguar Mk 2 wasn't introduced until late 1959.
Continuity mistake: At the beginning of the second part Michael returns to his wintry estate. When he enters the house a cigarette appears in his mouth. (00:01:05)

Other mistake: When Hyman Roth's assassin runs at the airport, he is turning around with one (of two) bullet holes already in his jacket before the sound of any gunfire. (03:10:50)
Audio problem: When Fredo is wakened by a phone call from Johnny Ola, the audio doesn't match up with Fredo talking. You can see his mouth moving at times when nothing is said, and talking that doesn't match his mouth movement's. Any idea what the original lines were?
Other mistake: When Michael visits Pentangeli's house, Pentangeli closes the double door to the study. There is a dead bolt handle on the right hand door. There is no deadbolt coming out of the door side, nor a striker plate on the opposite door.
Continuity mistake: When Fredo and Michael are talking at a Café, the red adjuster handle of the parasol is sometimes intact and sometimes it is broken. (01:28:10 - 01:31:10)
Chosen answer: They were not his actual grandparents, just people that knew his family and who helped Vito after his mother, father, and brother were killed. Vito refers to her as "grandmother" because they treated him like family. Their names are unknown.
raywest