Continuity: 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: After the beach scene, Costello is preaching young Sullivan on loaded guns. His cigarette disappears from his lips between shots.

Continuity: Just after Leonardo DiCaprio 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: When Billy and Frank are sitting in a bar, Billy is just sitting at the table. In the next shot he's suddenly in the middle of blowing smoke rings into the air.

Factual error: In one of the opening scenes, where Martin Sheen and Mark Wahlberg's characters are first interviewing Leonardo DiCaprio and Matt Damon individually, Mark Wahlberg refers to himself as a "staff sergeant." However, the Massachusetts State Police has not had staff sergeants since 1992.

Revealing: When Costello's men drive off, after throwing Queenan off the top of the building, the undercover FBI are firing at the van. One back window is shot out, then in the next shot of the van the other window is shot out and then the same one is shot again so the one window is shot out twice in the same scene.

Continuity: 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.

Continuity: At the end of the scene where Sheen and Wahlberg confront Nicholson on the street there is a man in a blue jacket and red tie positioned between Sheen and Wahlberg who was never introduced in the course of the movie nor explained within the context of the scene.

Continuity: 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: 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: After the football game, Sullivan turns to his right to call the other team "faggots". From a different angle he is making a left turn.

Continuity: When Sullivan crosses the street there's a lot of people going to the right. From the CCTV recording there's just one person.

Factual error: In the scene where 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: 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: When Leonardo is at the funeral near the beginning of the movie and reads the card signed "From Frank Costello," it is initially seen as a heavy bold script, but in the closeup, it is a thin script written in different handwriting.

Continuity: In the scene where Costigan is beating up the Providence goons, he is hitting the one in the cooler with his right hand. In the next shot he is hitting with his left.

Continuity: The dead woman on the beach lies with her feet together, but a shot later they are spread apart.

Revealing: When Mr. French smashes Costigan's arm cast, on the second attempt Costigan seems to be using his own force to smash the cast. As French loses his grip at the top of the motion, Costigan's arm moves downward with all the force it would have had had French not let go.

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 Alec Baldwin introduces Mark Wahlberg and he describes his undercover unit, a microphone is seen moving around at the top, center of the screen.