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.Sacha
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.
Revealing mistake: When Mr. French smashes Costigan's arm cast, on the second attempt Costigan seems to be using his own force to smash the cast. As French loses his grip at the top of the motion, Costigan's arm moves downward with all the force it would have had had French not let go.
Revealing mistake: In the scene where Costello is smashing Billy's hand with the boot heel, you can tell from the position of Billy's arm that Costello is hitting the pool table rather than Billy's hand. He is actually hitting his own hand more than he could possibly be hitting Billy's. (00:36:00)chiefs58
Continuity mistake: During the opening scene at the diner, Costello places a comic book in the grocery bag, and it is well below the top of the bag. In the next shot of the bag, the comic book is much higher in the bag, and in fact is almost rubbing the boy's cheek.
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.