Continuity mistake: When Ellerby attacks the guy for not installing the cameras in the right place, he knocks off a map from the board. A second later Ellerby is still going after him, and the map is fixed.
Continuity mistake: In the scene after Queenan dies, when Sullivan picks up his phone to contact Costigan, he presses the "talk" button and a smudge of blood is left. In subsequent shots, there's no trace of blood on the "talk" button. (01:49:45)
Continuity mistake: When Billy Costigan 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 mistake: When young Sullivan is at Mass, a priest is swinging the smoking incense burning. In the next shot the smoke is gone, and he is away, arriving with the burner and repeating the previous movement.
Continuity mistake: When Costigan is with Madolyn in her office, the tissues in the tissue box change consistently between shots.
Continuity mistake: At Billy's mother's funeral, the signature of Frank Costello on the flowers changes between shots.
Continuity mistake: 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.
Continuity mistake: While Costigan is having coffee with Madolyn, a coffee cup suddenly appears in his hands close to his mouth, and the toothpick that was in his mouth a second ago vanishes.
Continuity mistake: In the scene where Alec Baldwin is briefing the cops on the micro processor bust, his shirt has distinct sweat patterns. These patterns change between scenes, back to original then finally almost dry. (00:59:55)
Continuity mistake: The top layer on their dessert changes when Sullivan and Madolyn are in the restaurant. Between shots, the top piece changes back and forth between flat and curved.
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".
Continuity mistake: When Frank and Francis executes the couple on the beach, the woman falls almost on top of the man's back. When it cuts her body has almost moved off him. Then in the next shot she is on top of the man again. (00:04:15)
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.
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.
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.
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.
Answer: The assumption is that Wahlberg learned of Damon's betrayal from the envelope given to Vera Farmiga, Damon's therapist girlfriend. It could also have been for revenge, but Wahlberg did know that Damon was the rat when he killed him.
Dandude