Continuity mistake: The photo of Cleric Partridge with Mary O'Brien is crumpled and folded when we first see it in the morgue and when Cleric Preston shows it to O'Brien. However, when Preston is sitting at his desk and looking at the photo by himself, closeups of the photo show it as smooth without the signs of wear and tear seen previously.
Visible crew/equipment: When Preston's apartment is being searched, he walks into his bathroom to find his hidden Prozium capsules. He shuts the door in the camera's face and when you see the interior of the bathroom (with the camera behind Preston) you can very briefly see the cameraman's head.
Continuity mistake: In the final rampage through the corridor, Preston shoots through the door, killing the guards on his side but also those in the room - we see them falling to one side. But when the corridor guards fall through the doors, in shots from Preston's point of view the guards are visible standing just inside the door before being knocked down.
Continuity mistake: In the opening action sequence, one of the sense-offenders slams wide the front doors of the hideout and begins firing at the police. The camera cuts to the police, who immediately open fire; when we cut back to the sense-offender being riddled by bullets, the hideout doors are securely closed behind him.
Revealing mistake: There is a shot right after the first time the clerics are entering the city from the nethers that you see an APC patrolling the wall road above the entrance. In that shot two shadows are being cast on the road. As the CGI APC passes through them they simply don't cast a shadow on it as they should.
Continuity mistake: In the scene after the destruction of the "TV center", Preston is looking out through a window over the city's explosions taking place. In one shot his right hand is shown holding the red piece of fabric he got from Mary O'Brian. In a scene before that, when he destroyed the "TV center", his right hand was bloody. He could have gone to wash it in between, but that seems unlikely, since his neck wound is still not taken care of.
Continuity mistake: The book Sean Bean takes is quite narrow when it's in his pocket, but when we see him reading it later (definitely meant to be the same book), it's about 50% thicker. It was also originally a hardback, but when he's reading it he's bending the cover like a paperback. After he drops it, it becomes a hardback again.