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.
Revealing mistake: In the scene where Costello is smashing Billy's hand with the boot heel, you can tell from the position of Billy's arm that Costello is hitting the pool table rather than Billy's hand. He is actually hitting his own hand more than he could possibly be hitting Billy's. (00:36:00)
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.
Continuity mistake: When Sullivan and his pal are watching the golden dome building from a bench, from a back shot his friend has his head faced straight, but from the front shot it's sideways.
Continuity mistake: The dead woman on the beach lies with her feet together, but a shot later they are spread apart.
Continuity mistake: During the opening scene at the diner, Costello places a comic book in the grocery bag, and it is well below the top of the bag. In the next shot of the bag, the comic book is much higher in the bag, and in fact is almost rubbing the boy's cheek.
Factual error: In the 'She fell funny' scene, where Costello shoots the young couple, the aircraft that flies by is a 747-400, which first flew in 1988. The scene is set in 1986.
Continuity mistake: When Costello is hitting Costigan's broken wrist, as Costello is leaving the room, the shot shows Costigan crumpling to the floor beside the pool table. He is on the floor as Costello is leaving. But, the next shot shows him back on the pool table crumpling to the floor again from a bird's eye view.
