Revealing mistake: In the scene where Bud is holding the DA out of the window you can see a wire attached to the DA's legs to stop him from falling.
Factual error: In the scene where Lt. Exley arrives at the Night Owl after the murder, the patrol officer is wearing a Safariland velcro-fastened handcuff case on his left hip. That style of rounded cuff case was not introduced until the early to mid 70s.
Audio problem: When Bud White goes into the records room to 'handle' Exley, he begins to beat him. When Exley pulls out White's handgun and points it at him you hear him cock it. The next shot is of him holding the gun but the gun isn't cocked.
Continuity mistake: It's the scene where Russell Crowe is holding the D.A. out of a window. On the wide shot, he is holding only one of his legs, and the other is flailing about. But in the tight shot, he is holding on to both of his legs.
Continuity mistake: When Bud White pulls into the driveway at Pierce Patchett's house to question him about Susan Leffert's murder, the wind is causing a tree top (top center of the frame) to sway. Before Bud opens the car door, you can see the tree come to a sudden stop in a position different than that of the last frame.
Continuity mistake: In the scene where Lt. Exeley resets the clock, the first time you see the clock it is exactly 5 minutes after 2. When Exeley puts on his glasses to see the time, it is clearly only 3 minutes after 2 oclock!
Deliberate mistake: When White almost fights with Exley in the street and is pulled away, the public mailbox by the side of the road is all blue. Mailboxes back then were red on top and blue on the bottom.
Other mistake: When Jack Vincennes goes to the hotel to stop actor Matt Reynolds from sleeping with the DA, he finds Matt's dead body on the floor with his throat slit. In the closeup of Matt's face, you can clearly see the living actor's pulse in the left side of his throat.
Continuity mistake: Exley has gauze or cotton stuffed on the left and right side of his upper lip (a la The Godfather). It's supposed to look like swelling from the unbelievable beating he just took. If you look at him closely in these several shots, the amount of cotton in his lip changes. Then later the bruises are only on his left cheek, with no swelling of his lip.