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.
Other mistake: After the third homicide, everyone gathers around the statuettes of the ten little Indians. Small problem, though. There are eleven little Indians. (00:42:20)