In the scene where Det. McKinney and Dr Fowler are reviewing the unedited video of the shooting involving Officer Sullivan, the dialogue from Officer Sullivan is not consistent with the dialogue from the original shooting scene from earlier in the movie.
Near the end of the movie, with the detective and the doctor riding in the ambulance, the first head-on shot of the ambulance clearly shows the ambulance has a headlight out on the driver's side (3 lit). Subsequent shots show the ambulance with no headlights out (4 lit).
A mattress is clearly visible at the bottom of the frame when the policewoman jumps down from the roof during the hostage situation.
At the end of the movie with Maniac Cop chasing after the detective in a police car while being set on fire, you can see the safety goggles under his mask in several of the changing shots in this stunt scene.
When Detective Sean McKinney passes through a metal detector after handing his gun to a security guard, the metal detector does not trigger for the metal in McKinney's badge.
When the two detectives arrive at the crime scene of the decapitation victim, they step over the obvious dolly tracks at the bottom of the frame, which the camera then uses to move in.
Several recognizable landmarks in Los Angeles can be spotted in the film, despite that it's supposed to take place in New York City.