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.
Among the items in Ida Session's pocketbook, which Gittes rummages through is a social security card which is 1970s style with blue and red ink, not the 1937 style which had only one color, and a different look.
In scene with Gittes, Mulvihill, and Yelburton ("Mulvihill! What are you doing here?"), elevator call buttons are modern, automatic elevator type - they have lights. In the 1930's, elevator call buttons were generally black and had no lights.
When Mrs. Mulwray drives away from the Coroner's office you can hear a siren in the background. The siren is a modern electronic one. In the 1930's, they only had mechanical sirens which have a different sound.
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 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.
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.
When the "Man with Knife" (Polanski) 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 Polanski holding the knife the proper way.
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 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.
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.
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 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' 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.
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.