Revealing mistake: 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.
Revealing mistake: As Evelyn Mulwray drives Jake home after his altercation with the farmer look closely at the side window next to Jake. The car appears to be driving down the road but the reflection in the vent window is stationary. (01:13:40)
Revealing mistake: When Jake talks with a boy on a horse at the bottom of the LA River, in the distance you can see a television antenna on top of a house. There were no television antennas in Los Angeles in 1937. (00:39:45)
Revealing mistake: When Roman Polanski cuts Jake's nose at the Oak Pass Reservoir, looking closely, his nose is fine and the blood is just on his cheek. (00:42:35)
Revealing mistake: When Jack Nicholson slaps Faye Dunaway, the first time we can see he doesn't actually hit her. We see a shot of him raising his hand, then a shot of her jerking her head like she was slapped, but his hand doesn't touch her, or even go anywhere near her.
Revealing mistake: When Jake returns a second time to talk to Russ Yelburton they go into Hollis Mulwray's office and a lot of the pictures have been taken off the walls. The crew makes a very lame attempt, using spray paint, to make it look as though the paintings have been hanging on a wall for very long time. Earlier in the movie we see these same spots and there is obviously no discoloration on the walls. (00:55:55)
