Continuity mistake: In the final scene of the movie, when Sullivan is walking down the hallway and into his apartment, his belt is light brown. But when he's dead on the floor, it's a dark brown, almost black.
Continuity mistake: At the very last shot of the movie, taken from inside Sullivan's apartment, the Massachusetts State House appears to be quite closer than in other shots from the apartment - e.g. when Sullivan first rents it.
Continuity mistake: When French finishes slamming Costigan's arm on the pool table to break off the cast, one small piece remains that Costigan removes, leaving his arm bare. When Costello hits the arm with the shoe, there is again a piece of cast on Costigan's arm and a large piece still attached in the overhead view showing Costigan on the floor.
Continuity mistake: When Frank asks for "numbers", Madolyn is moving into the apartment. In the next scene, when Costigan and Madolyn "get together" she is shown as moving out, but it seems that Costigan was giving his numbers then leaving the hideout to go get with Madolyn.
Continuity mistake: At the end of the scene where Costello smashes Billy's hand, he pulls out a wad of cash with his left hand, peels three bills off with his right hand and throws it on the table, saying "Get your hand taken care of," then in the next shot, has the wad in his right hand and throws that down on the table, and the three bills he just dropped are no longer on the table.
Continuity mistake: When Sullivan is checking the CCTV recording, we can see the "citizens" envelope, lying on his coat, on a desk or a chair. First, the folded part of the envelope with "citizens" written on it is upright, with the word appearing upside-down. Then, as he grabs his stuff to leave, the position of the envelope has changed to horizontal. (01:30:00)