Continuity mistake: When Costello and French encounter the cops, the first shot shows the cops' car turning their lights on. In a new shot a second afterwards the lights on the middle and right side cars are out and they are turning them on again. On yet another new angle, the lights on the car on the left turn from off and on again in a split second.
Continuity mistake: In the beginning when Costello is saying: "You could become cops or criminals", he has a cigarette in his mouth, but when he leans into to say: "whats the difference?", it disappears.
Continuity mistake: In the scene after Queenan dies, when Sullivan picks up his phone, the number Sullivan calls Costigan in isn't the same as the one from which Costigan calls back, even though it's the same mobile (silver and blue vs. Costigan's red phone for Costello's gang). When Sullivan calls, the number begins with something like "617-854" and when he sees Costigan calling back, it's "617-177".
Audio problem: When Colin answers Costello's phone, we hear the girl ask, "Where the hell are you?" but her mouth actually asks, "Where the f*** are you?"
Continuity mistake: During the final shootout between the baddies and the cops, the pilot window of the grey GM is destroyed but appears perfect a second later.
Revealing mistake: In the shootout at the elevator near the end of the movie, when Sullivan shoots the last guy, you can clearly see kneepads under his pants, to soften the fall.
Continuity mistake: While Queenan is talking to Costello out on the street, watch the people behind him. First there's no one, then we have a shot of a woman and a huge line of kids passing next to her. Then back to Costello and the woman, who hasn't moved, is now miraculously visible behind him. To add to the mess, the kids have vanished.
Visible crew/equipment: When Costello is looking at the dead corpses on the beach, two beams of light get reflected on his glasses.
Revealing mistake: In the Porn theater after Frank reveals the dildo to Sullivan and they argue about it, when the camera zooms out and the view is behind Sullivan the guy to the far right looks like he was added post production.
Visible crew/equipment: When the song starts, right before the main title, we have a dolly shot of Billy in jail. It starts from a red wall and moves towards him. If you look closely at the wall, you can see the subtle shadow of the camera and the rest of the equipment as it moves.
Continuity mistake: When Costello's bad guys are pulling up in the van to the building where Queenan and Costigan are talking, you can see the wet tire marks where the van stops before it gets there.
Continuity mistake: Before French dies he starts to cough and there's suddenly less bullet holes in the windshield than in the previous shot.
Continuity mistake: When Queenan is interviewing Costigan, the metal thing and the papers around it keep changing positions depending on the angle.
Revealing mistake: 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.
Factual error: In one of the opening scenes, when Queenan and Dignam are first interviewing Billy Costigan and Colin individually, Dignam refers to himself as a "staff sergeant." However, the Massachusetts State Police has not had staff sergeants since 1992.
Continuity mistake: After the rugby game, when Sullivan turns around to insult the other team, there's a guy with a light blue shirt that steps ahead of another one with a dark blue shirt. A second later their positions swap.
Continuity mistake: After the beach scene, Costello is preaching young Sullivan on loaded guns. His cigarette disappears from his lips between shots.
Other mistake: When Anthony Anderson is shot in the head towards the end of the film, you can see him still breathing as he lays on the ground.
Suggested correction: It's unsettling, but sometimes a lethal head wound leaves the base of the brain that controls autonomic functions like heart beat and breathing intact. These victims may wheeze, bleed profusely, or even convulse vigorously for several minutes before expiring.
Continuity mistake: When Costello is getting his money in the diner, young Sullivan is first sitting next to some packages on the right of the screen , but a shot later, when he stands up holding a brown paper bag, he is seen sitting next to to an empty seat.
Continuity mistake: When Billy and Costello are talking in the bar, after Costello fires the paper cloth, smoke is visible in the shots involving Billy, but not in the ones involving Costello.
