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: When Gittes goes to Evelyn's house before she tells him the truth about her and Katherine, one of her servants opens the door and you can see in that precise moment the shadow of the camera on the door jamb on the left and on the wall next to the stairs while approaching to the actors. (01:47:23)