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: 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)