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.
Continuity mistake: After the general is killed, there's a close-up on the figurines. Two are broken with just the feet left, the third, the rightmost, at the waist. When the second victim was murdered, we saw the killer break just its head off, the statue to the right of the halved one. The following shot shows the statues in that order, with the addition of third broken one in rightmost position. (00:34:15 - 00:42:15)