Other mistake: In the original version of the film there's a mistake in the opening credits where Colin Blakely's name is misspelled as Colin Blankey.
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.
Visible crew/equipment: After Lightfoot steals the Trans Am, when he first encounters Thunderbolt on the run, he swerves into the field, and in a shot from inside the car, a piece of filming equipment on the hood of the car is visible through the windshield. Then Lightfoot hits Dunlop with the car. (00:07:50)
Revealing mistake: When Butt falls behind the suitcases, a mattress to soften the fall is visible.