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.
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.
Suggested correction: This isn't correct. In the big shootout towards the end, gunshots leave holes in car doors and break car windows from both sides of the fight.
Continuity mistake: While running from the cops, French turns the car 90º 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.
Visible crew/equipment: When Costello shows Billy the mutilated hand in a bag, filming crew members standing up are reflected on the wine glass.
Factual error: When Queenan and Costigan are riding the Red Line, Queenan gets on at Park Street station. Costigan tells him, "Get off at the next stop, South Station." South Station is two stops away; the next stop after Park Street is Downtown Crossing. However, Queenan does get off at the next stop, and it is magically South Station.
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.
Character mistake: During the funeral at the end before the 21 gun salute, the NCOIC called for "half left face", that would cause everyone to get shot in the face.
Suggested correction: Half-left face means a 45° left turn (left face is a 90° turn), so wouldn't result in anyone getting shot.
Revealing mistake: Costigan carries a Walther PPK/S. The hammer on his pistol is down both before and after he shoots the junkie armored car thief. The shot to the leg should have automatically recocked the hammer.
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.
Factual error: When Costello throws the cocaine powder and says "Want some coke? Don't stop till you're numb" the pupils in his eyes are constricted. Cocaine, being a stimulant, dilates your pupils. Constricted pupils occurs with opiates - narcotic analgesic.
Suggested correction: It isn't Costello taking the coke. He throws it on the ladies.
Visible crew/equipment: When Queenan dies after being thrown out of the building and his body is crushed on the ground in front of Costigan, as he looks at his hand covered in blood, in the shot on the top on the left side of the screen the dolly track of the camera is visible for a second.
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?
Factual error: When the MSP close in on Costello in Sheffield, they come in silently but still have their parking lights on. Pretty useless if you're "sneaking up" to a drug bust. Secondly, patrol officers don't have automatic weapons, as shown in the shootout.
Suggested correction: The parking lights thing isn't so useless. They're so close at the point we see them; anyone on lookout would see them whether they had their parking lights on or not. And it might be that those cars don't have a setting for no lights. My car doesn't; it only has a toggle for parking lights / headlights. No off.
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: 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.
