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.
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.
Suggested correction: This isn't a mistake. It is a framing scene using angled lenses to make the dome loom larger in the shot for poignancy. The whole point is to show both the close Rat and the far off dome in shot together in focus. Saying it is a mistake is not a fair judgement of the scene as it's not like they could have moved either of the buildings.
Continuity mistake: While running from the cops, French turns the car 90ยบ to the left, but from a different angle it's still facing in the original direction.
Continuity mistake: Costello tells Costigan "your dad had his own code". Then Costigan turns his head and exhales some smoke, but the following frame shows his head in the opposite direction and the smoke is gone.
Continuity mistake: Costigan enters the bar and points at a guy with a leather cap. In the background, a waiter is leaning on the bar, who then stands up and starts to walk. In the following close shot he is back in the previous place, leaning.
Continuity mistake: After the rugby game, Sullivan turns to his right to insult the other team. From a different angle he is making a left turn.
Continuity mistake: While Sullivan is rain drenched in military school, when he says "Sir! Yes sir!" his hair style changes in a nano second.
Continuity mistake: When Dignam (Mark Wahlberg) reports the processor theft to the SIU detectives, he says 20 chips were stolen. When Fitzy opens the case at the sale, there are 66.
Continuity mistake: When Jack Nicholson approaches a young Matt Damon he has a cigarette dangling from his mouth. When the camera faces Nicholson he has no cigarette. (00:04:55)
Continuity mistake: When a shootout between Sullivan's men and Costello's starts after Queenan's death, one shot initially hits the left glass of the van, then in the next shot the glass is intact, only to be hit again.
Continuity mistake: When French finishes slamming Costigan's arm on the pool table to break off the cast, one small piece remains that Costigan removes, leaving his arm bare. When Costello hits the arm with the shoe, there is again a piece of cast on Costigan's arm and a large piece still attached in the overhead view showing Costigan on the floor.
Continuity mistake: When Frank asks for "numbers", Madolyn is moving into the apartment. In the next scene, when Costigan and Madolyn "get together" she is shown as moving out, but it seems that Costigan was giving his numbers then leaving the hideout to go get with Madolyn.
Suggested correction: I don't understand why this is a mistake. People often take multiple trips when moving. Especially as she says she is keeping stuff at her old apartment due to having three weeks left on the lease. Also, if Costigan is leaving to meet up with her, why is this a mistake. She is his only friend, what is wrong with him wanting to meet with her?
Continuity mistake: At the end of the scene where Costello smashes Billy's hand, he pulls out a wad of cash with his left hand, peels three bills off with his right hand and throws it on the table, saying "Get your hand taken care of," then in the next shot, has the wad in his right hand and throws that down on the table, and the three bills he just dropped are no longer on the table.
Continuity mistake: In the restaurant, after Costigan says "School is out," he starts to bring the cigarette to his mouth, but before he smokes, while the action, music and dialogue continues, the shot cuts to him already exhaling the smoke.
Continuity mistake: When Costello lies dead in the shovel of the excavator, his right arm moves between shots. (First it is up - in the far shot, it is down).
Continuity mistake: When Sullivan is checking the CCTV recording, we can see the "citizens" envelope, lying on his coat, on a desk or a chair. First, the folded part of the envelope with "citizens" written on it is upright, with the word appearing upside-down. Then, as he grabs his stuff to leave, the position of the envelope has changed to horizontal. (01:30:00)
Continuity mistake: Costello throws money down on the pool table and tells Costigan to get his hand fixed. In the next shot he has the money in his hand and throws it down again.
Continuity mistake: When French is holding the bag with the hand he places a ring very close to Costello's cheek. A second later, without him having moved, the hand is at jaw level and farther away.
