When they enter the bank, there is a calendar that looks to say September. Even if it does say August, it starts on Sat. the 1st, putting the 22nd (the particular day) on a Saturday. In the 1970s very few banks were open on Saturdays, much less until 3 p.m. those Saturdays they were open.
When Sonny asks the bank manager to open the door on the bank vault, he notices the manager is using the wrong key. He takes the key from the manager and holds it in his left hand. The key disappears and reappears in his hand between shots.
When the doctor speaks to Sonny after his phone conversation with Leon, he begins to reach for the phone in one shot, but in the next his arms are at his side. It is in the second shot that he reaches when he is supposed to.
When Sgt. Moretti and Sonny are negotiating for the cops to bring pizzas, soft drinks, and aspirin, Moretti is holding a bullhorn in his right hand. In the next shot, he's gesturing with his empty right hand. Then the bullhorn is back in his right hand again.
When Sonny is having the tellers empty the money from the cash drawers, one of the teller's name signs says "Maria Sandora." When the telephone rings and the scene cuts to the manager and back to the tellers, the sign has disappeared. After the camera cuts to a different view of Sonny and the tellers walking to the next window, the sign is no longer where it was but is now located at this next teller window. After this, as the camera angles cut back and forth, the scene was obviously filmed at different times because the items on top of the counter are in different order and do not always appear in all shots.
When Sonny first meets the FBI agent he holds his hand up to shield his eyes from the light, when the camera cuts to the FBI Agent from the back of Sonny's waist, his hand is immediately at his side. When we see Sonny's face again he is raising his hand to shield his eyes from the light once again.
In the opening scene, when Sonny is walking to the bank, he
holds his gun in some gift wrap. The blue ribbons are crossing where the flower is. In the next shot, the crossing is moved much closer to the top, as if to make it easier to open the box. But when Sonny tries to open the box to get the gun out, the ribbon is back to its starting position.
Sonny tells the police that they are both Vietnam veterans. Later Sal says that he's never been on an airplane before. Some people believe this to be a goof, but later in the film Sonny's mother refers to his time served in Vietnam and also Sonny says he is "entitled to" a military funeral. It may be that Sonny is a Vietnam vet and Sal is not, or that both men are vets and Sal simply has not flown before and rather traveled via ship.