Continuity mistake: When Costello and Sullivan are in the porno theater, Costello's line is "it won't be me who suffers for it."When the tape is played back to Madolyn later, it says "it won't be me who pays for it."
Continuity mistake: When Costello tells the girl he has a hard-on, the position of her fingers on the blue book keep changing.
Continuity mistake: While having dinner, from a side shot we see Sullivan's mouth chuckle. Frame later, from the front, he is serious and smiles half a second afterwards.
Continuity mistake: Just after Billy Costigan is shot, the elevator doors try to close. In the shots where the black guy comes out of the other elevator, the doors aren't moving anymore.
Continuity mistake: In the scene in a bar where Costello talks to the young Colin Sullivan, the time in his watch is evidently very different in two shots (when he takes the money from the bar owner and later when he hands Sullivan a bunch of coins). First it reads 2:50, then 2:58, and only one and a half minute has passed in the movie, as shown by Costello talking. Also, the watch in the second shot has no seconds hand, only hours and minutes. (00:03:20)
Continuity mistake: When Sullivan is analysing the Providence corpses, the close-up shows both of his hands together, but on the wide shot one is near the corpse, and the other one close to his hip.
Character mistake: When Ellerby tells Sullivan "Some people don't trust a guy with an immaculate record. I do. I have an immaculate record." Really? An immaculate record even after you punch they guy's lights out for not installing the cameras right? He's a captain, had a long career, if he punches a guy for that, certainly he's had other complaints before too.
Suggested correction: This is too presumptuous to be a valid mistake. We know practically nothing of his history, so it can't be definitively said he's had complaints about him before. The assault he committed over the failed sting could very well be his first infraction, and if it is still being investigated, it won't appear on his record yet.
Continuity mistake: After Costello smashes Costigan's wrist he hands all of the dollars he has in his right hand. A frame later money reappears in the hand.
Continuity mistake: When Billy is sitting inside Queenan's house, the glass of water is in his hand in one shot, disappears in the next and then returns.
Continuity mistake: The position of Madolyn's spoon while eating the chocolate cake is not continuous in the front and back shots.
Factual error: When Costigan is riding the "T" (Boston subway), he is talking to Queenan on a cell phone. However, they are on the "Red" line, which is obvious because of the red walls in the tunnel. Cell phones don't work on the Red line because it is underground and there are no cell phone routers in those tunnels.
Continuity mistake: When Costigan is with Madolyn in her office. Madolyn doesn't have jacket on but when she runs out after him to give him a prescription she has a jacket.
Continuity mistake: When Sullivan is questioning a bad guy, he sits down with a cell phone right in front of him. But later on, he reaches out to our left to grab it.
Continuity mistake: After Costigan comes out of jail he visits an old lady. From a front shot she is slightly leaning, her left hand unseen. From a back shot the left hand is standing out over his shoulder with a burning cigarette. It should have been visible from the opposite shot at first.
Continuity mistake: When Sullivan crosses the street there's a lot of people going to the right. From the CCTV recording there's just one person.
Continuity mistake: In the final scene of the movie, when Sullivan is walking down the hallway and into his apartment, his belt is light brown. But when he's dead on the floor, it's a dark brown, almost black.
Deliberate mistake: In all the shoot-outs, the bullets that hit the cars spark, which bullets do not do, and leave no hole, dent, or even a small scratch. They do, however, break the glass.
Visible crew/equipment: When Queenan is in the subway and looks to see if he has a tail following him, you can see a boom mic appear and disappear in the top right corner.
Continuity mistake: At the very last shot of the movie, taken from inside Sullivan's apartment, the Massachusetts State House appears to be quite closer than in other shots from the apartment - e.g. when Sullivan first rents it.
Visible crew/equipment: When Ellerby introduces Dignam and he describes his undercover unit, a microphone is seen moving around at the top, center of the screen. (00:23:10)
