Continuity mistake: In the scene where Napolean has saved the girl's life in the cell blocks, they go back into the main part of the police station and Napolean holds the door to the cells closed. The first time you see him holding the door closed there are bullet holes above his head but there has been no gunfire and he does not act as if the people behind the door are shooting at him. It's not until the film cuts back to him a second time (or possibly the third) that there is gunfire on the sound track, the bullet holes are focused on and his character ducks down whilst complaining that he can't hold the door for much longer.
Factual error: In the scene where Julie, the secretary, has been discovered dead; the lieutenant approaches her, kneels down, and checks for a pulse with his thumb. As anyone with any medical knowledge should know, if you attempt to take someone's pulse with your thumb, you will get your own pulse and not the victim's.
Plot hole: In the final scene where they all leave along the passage; the father rolled out on a stretcher and the others casually walking out, you can tell by their smooth passage along the corridor that all the dead bodies have been removed, but this would have taken at least half an hour if not longer, so it would make no sense in reality for them all to wait for all the bodies to be removed before they left as they would have been the first to leave due to the traumatic experience they had just been through.
Other mistake: Ethan Bishop says to Wilson in the hallway as they plan their last stand "acetylene tank, magnesium flares...I stand in the doorway with my rifle, I shoot the flares, they ignite the tank, we blow the hell out of everybody in this hallway" But why can't he just shoot the tank directly as that would ignite it? Then it would be easier as a bigger target? Just igniting flares alone would probably not ignite the tank anyway as they would not have enough force to pierce thick metal. As he said in previous scene about the tank "if a stray bullet had hit this..."