The sign at the Victory Motel indicates each room has a television. This is highly unlikely given the time the story takes place. Televisions were still rare, especially in a motel that was already closed in 1952.
In the street scene after the kidnapped girl is rescued, a blue U.S. Postal Service box is visible. Mailboxes were painted olive drab until the color scheme was changed to red, white and blue on July 4, 1955. The U.S. Postal Service did not exist until 1971, when it replaced the U.S. Post Office Department. Prior to 1971, equipment, including trucks and mail boxes, were labeled "U.S. Mail."
When the kidnapper of Inez Soto is watching TV just before White sneaks up on him and shoots him, the carton of milk on the coffee table is of today's "fold-open top" design, which was not invented until well after the early 1950s.
When Det. Sgt. Jack Vincennes is sitting in the Frolic Room he unfolds a 50 dollar bill which has a green 'FIFTY' on the right side and a "B" signifying the Federal Reserve office on the left. None of these symbols were used on bills from the early 1950's.
(at 1:12) Jack Vincesses (Kevin Spacey) uses the term 'high-class porn' to Sid Hudgens (Danny Devito). Per the OED, the term 'porn' was not common parlance. Vincennes would have called such fare 'stag films', and high-class would never be used to modify it. That is a late 20th century intensifier.
During several outside shots, especially during the raid on the kidnappers' den, downtown Los Angeles can be seen in the background featuring buildings that were not completed until several years later.
The interior of the Frolic Room is a decade out of date. It was remodeled in 1963. The iconic neon outside is from 1948, but the neon cocktail glass, originally part of the sign, had been removed by the time of filming.
The scene outside the liquor store where Bud White confronts Leland "Buzz" Meeks ends with White unloading a revolver he had earlier confiscated from Meeks. He spins the cylinder off-camera, heard as a distinctive series of rapid clicks. However, only old style single-action revolvers - the type seen in westerns - make this sound when their cylinders are spun. This gun is shown to have a swing-out cylinder, which makes no sound when spun like this, since the cylinder is disconnected from the lockwork while swung out.
Despite all the noise and commotion made when Bud White is dangling the D.A. out his office window, the District Attorney's conscientious secretary never once comes in to ask what's going on, nor does she call for help.
At the start, just after Ed Exley is interviewed, Edmund Exley and Dudley Smith walk down the stairs of the police station where Dudley picks up two drinks. He hands one to Ed and the other for himself. When Dudley walks away, his has disappeared.
During the shot of when Bud White's vehicle arrives at Pierce Patchett's house, there is a JUMP-CUT where several frames have been removed. This is noticeable just before Bud opens his car door, the trees in the background appear to suddenly skip in motion.
During the fight in the records room of the police station, Bud White inflicts several bruises to the right side of Exley's face. In the very next scene, Exley's face is clear, but when he shows up at Lynn Bracken's home, his face is bruised and discolored again.
When Bud White drives up to the Victory Motel for the interrogation of Sid Hudgens, it is raining heavily and the sky is gray and overcast. But in the brief shot of Bud running through the doorway, the parking lot in the background is bathed in bright sunshine.
The blood on Exley's face is smeared one way when he is chasing Ray Collins down the hall and another way when he fires into the lift in the next shot, even though any blood would have spilled onto the lift door.
At the end of the movie, when Exley is being questioned, it can be seen that he has stitches on the right side of his face and a broken left arm. A few minutes into his questioning (at around 2 hrs mins) there is a close-up on Exley which shows both injuries being on the left side of his body. However, in the remaining scenes, it goes back to the injury (stitches) on his face being on the right side and the broken arm being on the left.
(at around 1h 45 mins) During Bud White's office fight with Ed Exley, Exley pulls White's revolver from its holster and hits him across the forehead. As Exley stands up, he clearly cocks the revolver's hammer as he points it at White. However, in the next camera shot, the pistol's hammer is uncocked.
When Bud White goes to visit Lynn Bracken and they sleep together, when he leaves the Victory Motel, it is dark. When he is waiting outside and she says goodbye to her "company", it is also dark. But as she goes around straightening up her place, the pink light from the setting sun is coming through the windows, and when she opens the door to Bud, it's twilight outside.
In the fight scene which takes place in the file room when Bud White confronts Ed Exley with the photographs of him with Lynn, there are table lamps on the desk that are lit up. During the scuffle, one of the lamp light bulbs burns out and once the scuffle is over, the light is back on as if the bulb didn't burn out.
During the fight between the cops and the Mexicans, the newspaper photographer raises his camera to take a photo. When he does so, the flashbulb fires. However, the resulting photo was obviously not taken with a flash. If it was, the officers in the foreground would have been overexposed and the rest of the scene would have looked more as though it were lit from the front, rather than from the sides and above.
In the early drug bust scene with Vincennes and Sid, a 1950 Ford police car is sitting in the parking lot of the bust house. It has 1956 Ford hubcaps, which would not have been factually correct on a 1950 Ford, much less a movie from 1953.
The phone number of Fleur-de-Lis is CRestview 2239. At the time, Los Angeles was using a mixture of phone numbers with 4 and 5 digits after the exchange (what today would be called 6 and 7 digit phone numbers).
White leaves Lynn's house when it is raining. It is still raining when he fights with Exley in the records room, yet when they confront the District Attorney, it is clear outside with scant evidence of rain having fallen. However, these locations are far enough apart that this is possible. In addition, there are rain streaks and tire tracks that imply the rain had fallen there earlier.
When two of Mickey Cohen's henchmen are gunned down in their car (right after the scene where Bud White gets his badge and gun back from Dudley), the outline of an explosive squib can be seen on each actor's forehead before the gunfire begins.
A click is heard as Exley tries to pull the trigger again on his M1911-A1 while the slide is locked back on an empty magazine. The 1911's slide prevents the hammer from falling again in this condition so it doesn't make a click. The trigger just compresses a bit, silently.
The goof items below may give away important plot points.
In the final scene, two woman walk past each other, behind and to Ed's left, as he is standing on the sidewalk. However, when he is seen through the rear window of the car - from Bud's perspective - the original women have disappeared and two new women (walking together, toward Ed) have taken their place.
When Dudley walks toward the approaching police cars while holding up his badge, Exley is not standing in the motel doorway and the shot that kills Dudley comes from off-screen to his right. In the very next shot, Exley is standing in the doorway to Dudley's left, lowering his shotgun.
(at around 2h 5 mins) Ed Exley grabs the pump shotgun from a mattress and points it at Dudley Smith. He then cocks the shotgun. But Exley had been shot through the left shoulder, and his left arm is disabled. He grabs the shotgun by the stock and trigger with his right hand, and then the sound of him cocking it. A pump shotgun cannot be cocked this way with one arm unless holding the shotgun by the pump handle itself. (This was how Sarah Connor cocked her shotgun using one arm in Terminator 2.)
Lynn Bracken tells Bud White, "Sue [Lefferts] came on the bus with dreams of Hollywood. This is how they turned out." Later, White tells Johnny Stompanato that Meeks is "under a house in Elysian Park and he don't smell too good." The Lefferts' house is in a Los Angeles neighborhood, so Lefferts wouldn't have come on a bus. Brian Helgeland's original script placed the Lefferts' house in "San Berdoo," or San Bernardino, so Bracken's line would have been consistent before the location change.
At the Victory Motel shoot out, Dudley Smith is clearly seen shooting Bud White in the right cheek after he stabs Dudley in the leg. At the end of the movie when Bud White is sitting in a vehicle saying goodbye to Ed Exley, the bandage is on his left cheek. However, Bud also has clearly visible barely closed wound on the right cheek. Exit wounds are usually more severe than entry wounds so the bandage on the left side on the face is justified, assuming that the bullet penetrated both cheeks.
(at around 1h 50 mins) When Bud White is dangling D.A. Loew out of a window, a wire is visible going to the D.A.'s feet. (There is no visual shown of a phone and no resolution given to suggest anything other than a safety harness.)
When Bud White rushes into the morgue to find Dick Stensland's body, one of the sheets he turns up uncovers an older man with grey hair who appears to have almost had the skin scraped off of him (or maybe burned off). In a later scene where Exley is speaking to the coroner about the identification of another corpse, an event depicted as occurring days later, the coroner is doing an autopsy on the same body. It's obvious that the production crew simply recycled the body, but in reality, it would have been a grievous error for a coroner to wait so long to do an autopsy.
At the end of the Victory Motel shoot-out, when Exley stands next to Captain Smith's dead body while holding his badge up, James Cromwell's body is clearly seen moving up and down as the actor is breathing.