Continuity mistake: In the scene where Tommy shoots Spider in the foot, we hear 7 gunshots from a S&W Model 36 5-shot .38 revolver. [The first gun, not the Colt 45 he kills Spider with later.]
Continuity mistake: When Tommy kills Stacks he shoots him once in the back of the dome, then cracks Carbone's castanets about the coffee. Then it cuts to a slo-mo reprise of the whacking with a Henry voice over and Tommy shoots 6 times.
Continuity mistake: When we see the close-up of Tommy pulling the ice pick out of Morrie's head, there's lots of blood on the ice pick. But in the next shot, there's no blood on the ice pick. The next shot is a little quick and blurry, but we can still easily see the ice pick's silver when it should be red.
Continuity mistake: After Karen has been with her girlfriends, she is talking to Henry about what is going to happen if he gets killed, at that point Henry is folding his pants by the closet, when the camera angle switches he is suddenly very quick on his knees in front of Karen.
Continuity mistake: When Henry and his girlfriend enter the Copacabana avoiding waiting in the line, their way through the catacombs of the nightclub is prolonged artificially because they exit the kitchen by the same door they entered it. The scenery is changed a bit by moving some racks and a curtain but you can tell it's the same because they pass the same firehose on the wall twice.
Other mistake: When we see the shot couple in the pink car, there's blood spatter inside the windows and windshield, so that means the windows were up when they were shot, and they were shot in the car. It makes no sense that neither the windshield nor the windows are broken. If Jimmy shot them from the front, he would've had to have shot them through the windshield. If he shot them from the side, he would've had to have shot them through the window.
Continuity mistake: When Karen is at the the Hostess party, and Jimmy Conway's wife is talking about Jeanie's family, if you look at the lady she's applying make up to, she crosses begins to herself twice. One when the camera is one her and Jimmy's wife, then again when the camera is just on her.
Factual error: When Henry meets with Jimmy after his arrest, the scene takes place in 1980, yet a 1984 or newer white Corvette is seen parked across the street at the end of the scene. (02:14:50)
Continuity mistake: Paul and Jimmy go over to Henry's girlfriend's place with Henry, and she leaves to get cigarettes. Paul is talking to Henry about having to go back to his wife, and when they sit down, the camera goes on Henry and you can see a Jewish necklace. But later when the camera goes back to Henry, it's a necklace of a cross. (01:16:15 - 01:16:50)
Continuity mistake: When Henry beats the neighbor kid and then passes the gun to Karen, her knit sweater is sometimes on one shoulder and sometimes on both arms.
Continuity mistake: After Tommy shoots and kills Spider, Henry sits back up and puts his hands down from covering his ears twice.