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.
Continuity mistake: In the scene in a bar where Costello talks to the young Colin Sullivan, the time in his watch is evidently very different in two shots (when he takes the money from the bar owner and later when he hands Sullivan a bunch of coins). First it reads 2:50, then 2:58, and only one and a half minute has passed in the movie, as shown by Costello talking. Also, the watch in the second shot has no seconds hand, only hours and minutes. (00:03:20)