Continuity mistake: In the scene when Il Duce approaches the house before he begins shooting at the three, he steps in front of a car without a hubcap. During the battle, you can now see a hubcap on the wheel. (01:17:05)
Visible crew/equipment: In the firefight scene, when Rocco and the brothers walk out of the house and see Il Duce, crew members are visible in the sunglasses. The most prominent example is in the head-on shot of the three men with Rocco in the middle. A man in a red jacket holding the camera is visible along with a half-dozen other crew members. Several of the shots between the timestamps have crew members' reflections visible in the glasses. (01:17:45 - 01:18:15)
Continuity mistake: After Connor has thrown the toilet off the roof onto one of the Russians and landed on the other, he is unconscious on the ground. While Murphy is collecting things from the ground and the Russians, Connor's legs switch from straight to crossed. (00:24:09)
Continuity mistake: In the Don's house toward the end of the film (just after the saints are captured in the basement) the Don lights up a cigar and says he's leaving. When he steps outside and gets into his car, you can see a cigarette in his mouth, not the cigar he lit.
Factual error: When the news reporter is at the Lakeview Lunch which is supposedly on Commonwealth Avenue in Boston, below her name it says South Boston, as if that's where they were reporting from. (01:04:10)
Other mistake: In the scene where the brothers are going to kill the "fag man", the fourth man has his shoulders perpendicular to the brothers in the center of the room but the bullet goes straight through his chest. The shot wasn't even at a close enough angle to go through his chest like that. (00:40:35)
Continuity mistake: When they bring coffee to the brothers in the police station, one of the guys holds up the tray and there are three cups visible. The brothers take two and walk away. The next shot is from in front of the brothers, where you can see, over their shoulder, that there is no one holding up the coffee tray. The next shot is of the group again, and the tray is back to being held up.
Continuity mistake: While Connor is struggling to rip the toilet free, one corner of the tank lid (front right, from Connor's perspective) crumbles and a few pieces break off. Then a few moments later, the chips have vanished and the lid is whole again. (00:22:35)
Continuity mistake: The bag in which Pappa Joe Yakavetta's sandwich arrives changes in different shots. When the camera faces him, the bag is angled toward him; when the camera is behind him, the bag is angled out, away from him. (00:29:00 - 00:31:30)
Revealing mistake: In the scene where Rocco kills the two 'goodfellas' in the restaurant, he hides his gun by holding the gun behind his coat. Before the shot switches, you see the gun go off, yet the hammer is down the entire time. The revolver could have been single or double action, but the hammer would still have to have gone back. It is easier to see the gun go off on the NC-17 version or on the deleted scene, but you do see the flash on the R version.
Visible crew/equipment: When all the news casters are trying to talk to William Dafoe, after he lets the second man in the door you can see a cameraman and a person next to him in the reflection of the door. (It is too close to be a news reporter).
Continuity mistake: In the shootout between Il Duce and the Saints, one of the brother's runs out of ammunition and the pistol locks with the slide back. The camera angle changes and immediately, without loading a new magazine, he fires off two more rounds.
Revealing mistake: Prior to the scene where the trio raids the card game, the three bring the wife, hands bound, to enter the security code. When the guy "cuts" the duct tape, look closely at the wife's hands, the knife never actually cuts the tape; rather he pretends to cut and simply lifts the tape... and voila, her hands are free.
Factual error: At the beginning, the MacManus brothers are attending a Catholic mass. The priest is saying the Our Father, and concludes it with "For thine is the kingdom, the power, and the glory for ever and ever." However, Catholics do not include this line when they say this prayer - only Protestants say this. Also, the prayer is never said before the priest gives his homily as it is in the movie.
Continuity mistake: In the scene where Connor is writing the hotel information down from the Russians, the position he is writing on the paper changes from directly underneath the last line to further down on the page back to directly underneath and further down again. (00:28:37)
Continuity mistake: Right as Agent Smecker shouts "There was a firefight," the Duke is shown briefly. Right before this frame, he (The Duke) already had his guns drawn, yet now his hands are down in front of him, coat closed.