Factual error: When the detectives are first bringing Calogero out to the street after the shooting by Sonny, an electronic siren can be heard winding down and cutting off mid-tone. Only motor-driven sirens were available on emergency vehicles during this period, and when they were turned off, they simply wound down.
Factual error: Although the movie neighborhood is meant to be 187th Street and Belmont Avenue, a NYC bus (from the old Surface Transportation System) is shown running along that street, where none ever did. In at least one scene, the sign on the bus indicates it's the 12 bus, Fordham Road from Sedgwick Avenue to City Island. Fordham Road was three blocks north of 187th Street, and never had its own bus route.
Revealing mistake: When C is rolling dice with all the guys, Jojo the Whale keeps trying to get a better view and keeps breathing on Sonny which gets Sonny mad. In one of those shots, you can see Jojo look up at someone in the production crew, behind the camera view and as if on cue, start to get close to Sonny and get him mad again. There is nothing behind the camera that's shooting the shot except a wall.
Continuity mistake: When Sonny is letting a young C shoot craps and tells Frank "Coffee Cake" to go to the bathroom because he doesn't want C looking at him when he rolls the dice. Coffee Cake is wearing a beige short-sleeve shirt and even though he is shown in the bathroom in the next scene when C is about to roll the dice, Coffee Cake's beige shirt and arms are shown at the end of the crap aisle as if he was never sent to the bathroom in the first place.