Deliberate "mistake": When Sullivan makes the phone call to Costigan using Queenan's cell phone, he leaves Queenan's blood on the talk button. It's been hours or even days since Queenan's murder, how was the blood still wet?

Other: In the scene where Costigan drags a handcuffed Sullivan into the elevator, Costigan slams Sullivan against the wall and his left arm is visible and not in handcuffs.

Continuity: At Billy's mother's funeral, the signature of Frank Costello on the flowers changes between shots.

Continuity: 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.

Revealing: ***SPOILER*** When Costello is killed, a blood-pack is noticeable under his shirt.

Continuity: 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: When Costigan is with Madolyn in her office, the tissues in the tissue box change consistently between shots.

Revealing: When Costello's hooligans drive away from 344 Wash after the shoot-out, Queenan's body on the ground is quite obviously a dummy.

Continuity: 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.

Revealing: 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: In the restaurant, after Costigan says "The Academy is over" he starts to bring the cigarette to his mouth but before he smokes, while the action, music and dialogue continues, the shot cuts to him already exhaling the smoke.

Continuity: 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.

Continuity: 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.

Visible crew/equipment: When Costello is looking at the dead corpses on the beach, two beams of light get reflected on his glasses.

Continuity: 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: After the football 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: 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: In the scene where 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.

Audio problem: When Matt Damon answers Costello's phone the girl asks "where the hell are you?" but her mouth says "where the F*** are you.

Continuity: 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.