Plot hole: If we assume that Dexter took off the oil cap while it was still in the shop for Arthur, it still doesn't explain how Dexter got the jump on him when his car (hours later) broke down. Arthur's not shown to stop, but to merely drive for hours on end until night. Arthur's on a barren strip of road with no other cars or traffic to be seen for miles. It cuts from Arthur looking in his engine to Dexter suddenly walking out from behind the Mustang. The only explanation is that Dexter waited in the car with Arthur, which doesn't make very much sense. All this takes place in the last 15 minutes of the finale.
Visible crew/equipment: Before Debra is taken away in the ambulance Dexter is physically stopped from getting closer to the scene, and when Batista tells the cop that Dexter is one of them, in the next shot facing Dexter the reflection of the moving overhead boom mic can be seen on the car window.
Revealing mistake: When Deb is texting "Looks like it" to Anton, the buttons she presses on the keypad do not correlate with the message itself and the phone doesn't show any auto-correct feature being utilized, indicating that the text message was added in post production.