When Gittes awakes and sits up on the ranch house porch after being being knocked unconscious in the Trident Ranch orange grove location, one can clearly see a circa 1974 refrigerator through the window behind him.
During water hearings in town hall, one spectator reads a Sunday newspaper comics section in which all the panels are clearly free-form Seventies style art, nothing like you'd see in comic strips of the Thirties.
When Jake finds the glasses in the Mulwrays' saltwater pond we clearly see a shot of a goldfish (possibly the Black Moor variety)- a freshwater fish that cannot survive in salt water.
Among the items in Ida Sessions's pocketbook, which Gittes rummages through is a social security card that is 1970s' style with blue and red ink, not the 1937 style, which had only one color, and a different look.
During the "Mulvihill! What are you doing here?" scene, elevator call buttons are modern, automatic elevator type with lights. In the 1930s, elevator call buttons were generally black and had no lights.
When Gittes takes photos from the rooftop of Mulwray and a young woman, the scene cuts away to a shot in front of Gittes, where his 35mm camera captures Mulwray embracing the woman on the terrace below. However, the image in Gittes's camera should reflect an upside down image of the couple. According to the interview with Roman Polanski on the DVD, he deliberately chose to show the couple right-side up to make it easier for the audience. He also said that "now" (in 1999) he would have shown it as upside down, as it would be in reality.
Police cars use the higher-pitched style of siren commonly heard from the 1960s onward, not the lower-pitched style characteristic of the 1930s.
They're in a drought and heat, yet when Jake enters the orange grove, fully ripe oranges can be seen. Oranges ripen in winter and are all picked before spring.
When Jake gets home after following Mrs. Mulwray and his phone rings, it is clearly not the sound of a 1937 telephone. The sound of his placing the receiver on the cradle is authentic, but the ringer is no earlier than the sixties.
When Mrs. Mulwray drives away from the Coroner's office, a siren is heard in the background. The siren is a modern electronic one. In the 1930s, they only had mechanical sirens, which have a different sound.
When Gittes visits the orange grove and crashes into the tree, you can see that a steel plate has been placed around the tree to protect it.
At the end of the movie, the cop shooting towards Evelyn's car starts stooping on his own, before Gittes coming from behind has a chance to grab him.
As Jake and Evelyn leave the nursing home, several cars from the 1970s can be seen passing on the road outside.
When Gittes gets in a fight with the owners of the farmland, one of the lenses of Gittes's sunglasses comes off. However, in the last shot Gittes is laying down with the sunglasses intact.
When Gittes first drives up to the Mulwray's home it's supposed to be lunchtime but the long shadows indicate this scene was shot in the early morning or very late afternoon.
When the knife man sticks his switchblade up the nostril of Gittes, the sharp side of the blade is positioned inwards, not outwards, and the cut is made with the dull side of the knife. The following reverse angle shows the man holding the knife the proper way.
When Jake is in the barbershop and has an argument with a banker he gets out of his chair to get in the banker's face. When he returns to his chair you can clearly see a reflection of a boom mic in the window in the background.
Gittes lights a cigarette while waiting to see Yelburton for the second time. Entering his office, he still takes a puff. But when they shake hands, the cigarette is gone.
In the orange grove scene, Gittes's car has its right front tire shot and deflated, yet it is not deflated when the car hits the tree.
Early during Jake and Evelyn's post coital cigarette, a mechanical noise is heard in the room - presumably the result of an incautious crew member.
The final scene takes place in Los Angeles' "New" Chinatown that opened in 1938, one year after the film's story takes place.
When Gittes and Evelyn Mulwray are arguing after lunch in the Biltmore Hotel's limousine lane location (while the valet fetches her Packard), the camera is facing west towards the 1926 Checkers Hotel at 535 S. Grand Ave. However, the camera is angled in a way that reveals a completely incongruous 1967, forty-two-story AT&T building (address is 611 West Sixth St) just to the south of the fifteen-story Checkers Hotel.
When Gittes gets drenched by the water, he's completely immersed in it. Then moments later his suit is fairly dry looking as though he had only been splashed.
When Cross and Gittes are lunching, the close-up of Jake's plate does not match, with the fish, potatoes and lemon wedge being in completely different positions.
The kitchen floor of Ida Sessions's apartment has vinyl sheet flooring in the "Armstrong No. 5352" pattern that was popular in the 1970s. However, the actual flooring with this pattern sold in the 1930s was linoleum, not vinyl, and pattern was inlaid and textured to the point where the it actually looked like grouted tile, as opposed the uniform flat surface of the vinyl flooring sold in the 1970s that is shown in this scene.
When Jake Gittes is about to enter the orange grove, he encounters a sign that says, "Keep out, no trepassing." Instead of saying "trespassing," the sign says "trepassing."
The clerk in the Hall of Records says some of the plot maps for the northwest San Fernando Valley are in Ventura county. The county line is well to the west in the surrounding hills.
When Jake first goes to Mr. Mulwray's office and lets himself in to search it (at 20 minutes), he looks through the desk's drawers. Upon opening the third drawer (the drawer before the empty one), a magnifying glass is visible (as is the red toiletries box he will remove). The camera cuts to his face, and when it returns to the drawer, the magnifying glass is suddenly covered by a booklet reading "Alexandria- Los Angeles". It was not there originally, but after appearing, it remains until the drawer is closed.
When Jake arrives at Ida Sessions's apartment at 848-1/2 E. Kensington in Echo Park, the trees in front of the building shift back and forth while the building does not, indicating a poorly executed overlay.
When Gittes and Escobar pull Hollis out of the channel at the reservoir, Gittes has on a brown suit with a tan shirt under it. A scene later, when Gittes and Escobar confront Mrs. Mulwray, Gittes has on a tie similar to Escobar's in the scene before with a gray suit. Escobar's suit and tie are also different.
L.A.'s original Chinatown was demolished between 1933 and 1936 to make way for Union Station. The current Chinatown, located a few blocks away, opened in 1938. So the only time L.A. had no official Chinatown was 1937, the year in which this film is set.
The streetlight shadows are inconsistent when Gittes places the stopwatches under Mulwray's tires.
When Jake tails Evelyn's car as she is going to the house where she stashed the girl, we can see that the sun is starting to come up, but when Jake parks in front of the house and starts nosing around, it is pitch black and clearly the middle of the night.
Reflected in the glass behind Gittes while he signs the contract with Mrs. Mulwray.
The bandage on Jake's nose after it has been split changes position from his office to when he meets Evelyn.
The tires squealing when Gittes pulls into the driveway of Katherine's house.
When Gittes takes Evelyn's letter from his pocket in the restaurant, the envelope has ECM in the corner. He asks her what the C stands for. However, in the sequel The Two Jakes, he looks at the letter again and at the top of the letter is printed "Evelyn Cross Mulwray". This means he would have already known when C stood for and there was no need to ask.