Revealing mistake: When Costello's men drive off, after throwing Queenan off the top of the building, the state troopers 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 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.Sacha
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: 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.
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.