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.
Trivia: The assault takes place on Precinct 9, Division 13. There is no "Precinct 13" in the film. At first John Carpenter wanted to call the film "The Anderson Alamo" (the original title of his screenplay), and at one point he changed the working title to "The Siege." CKK, the film's distributor, was responsible for the misnomer; it rejected Carpenter's titles and came up with the name "Assault on Precinct 13" (which it felt was more ominous sounding) during post-production.