When John and Clementine leave the motel for their honeymoon in Niagara Falls they are driving a Chrysler product, possibly a Plymouth Gran Fury. The Chrysler "star" hood ornament is clearly visible as they leave. When they are driving across the desert, a view out the windshield shows a blue-and-white Buick Regal hood ornament. The character Sydney is driving a silver/blue Buick Regal.
When Sydney and Clementine are drinking coffee in the diner, Clementine's cigarette switches from her right to her left hand between shots.
At 30: The craps table that Sydney walks by shows players cheering, but the cover (lid) and count slip are on top. If it were a real game there would be no cover.
Halfway through the movie the dealer calls out a 7 was rolled but the dice clearly show a hard six.
Camera operator's shadow visible on the ground during the first scene where Sydney meets John (full-frame version only).
When Sydney first walks to John sitting outside the restaurant, the camera tracks behind him. At the end of the shot, the camera's plastic cover (it was a damp day) is visible on the left in the door glass.
In the motel room, Sydney leans over to see if the hostage is breathing, and places his right hand on the bed headboard, after he has already wiped the room for fingerprints.