The sign above the police station door reads, "Division 14" instead of 13.
Film takes place in Precinct 9, Division 13, not Precinct 13.
In two police radio conversations the dispatcher refers to the unit he is talking to by one call sign; the officer replies identifying himself by an entirely different call sign.
While Kathy and her dad are trying to find their way to the nanny, Kathy spots a police car and suggests they ask the police for help. Looking out through the front window of the car, we see a deserted roadside with only the police car. Looking out the rear window a moment later, we see the police car in a row of other cars.
When the patrol car turns around at the station before the final assault the two officers are clearly seen wearing their uniform hats inside the car. Before and after that scene the officers are shown inside the patrol car with no hats on.
When a locked wooden case full of police rifles and shotguns is opened, they are already loaded. Law enforcement agencies load weapons intended for immediate use, but not weapons put in cold storage.
The silenced revolver that Wells uses would not be silenced. On the vast majority of revolvers have a gap between the cylinder and the barrel, through which some gas can escape, rendering the silencer ineffective.
Chaney's sergeant stripes are upside down on his uniform sleeves, which is generally indicative of a military way of wearing them, rather than a civilian law enforcement way of wearing them.
When Wells hot wires the car and speeds away, the director shows the audience a long insert of a car rapidly traveling a dirt road. The car is at least two blocks away before the gang member in the back seat shoots Wells with a silenced rifle. Yet Wilson, Bishop, and Leigh clearly see and hear the killing but rationalize the sound as perhaps a window breaking.
The term "Precinct" is incorrect for the police station. Los Angeles police stations are known as divisions, not precincts. Additionally, they are further identified by names, not numbers.
When Leigh, and moments later Wilson and Bishop, walk through the basement to the stairs to leave, they walk without having to avoid all the bodies that were there a few minutes ago. It's obvious a little bit of time has passed between the explosion in the hallway and the cops/medical personnel arriving on the scene. The bodies have very likely already been removed, this clearing the hallway floor.
At the prison, you can tell that the guard in the perimeter booth is likely a cardboard cutout versus a real person. The pose is exactly the same in both shots and unmoved as the bus leaves the prison.
When we see gang members cutting their hands, the man with black hat clearly didn't cut himself.
When Wilson holds the door closed to fend off the horde of thugs, the whole wall shakes visibly.
The little girl is still breathing after being shot which means she is not dead.
Julie keeps breathing, even though she was just killed. Her chest moves up and down.
Basement access to the sewer is used only to get one defender of the station to a manhole near a parked car. Obviously all the defenders could have escaped via the sewer, coming up in a location far from the station.
When the defenders of the station run low on ammunition, it never occurs to them to use the guns of fallen gang members.
When the shooting starts, Bishop makes the observation that neighbors would not have heard the gun shots coming from the gang members, as they were using silencers. He could have fired off a few loud rounds of his own, alerting anyone near by then.
Several characters talk about going to death row in Sonoma. San Quentin State Prison, the only mens death row prison in California, is in Marin County. Marin merely borders Sonoma County.