Revealing mistake: When Crewe dumps the car in the river, you can see the wire attached to it that was used to pull it out of the water. Burt Reynolds mentions it on the DVD commentary.
Revealing mistake: When Mr. Green pulls out of 28th Street, we see him disengage the "Dead Man's Feature" (by pressing down on the throttle with his hand). However, he does not actually move the control lever to make the train move. In that position, the train is at "idle" and will not accelerate.
Revealing mistake: When Jackie shoots her brother's left ear, there is a hole punched into the wall, behind him. We should be able to see into the hole, but we can't because the angle of the hole into the wallboard is wrong. From where Jackie took her shot, the bullet should be angled similarly to the (extra bright) light and the camera angle (our POV), but, instead, it goes perpendicularly into the wallboard. (00:35:20)
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 Clint Eastwood and Jeff Bridges are being chased by George Kennedy and his partner, Bridges is driving off the dirt road down and embankment and you can clearly see that in the passenger seat is a faceless dummy instead of Eastwood or another stunt man.