When Watty is driving with Starlene next to him, as they begin their trip to Mexico, in the shots from one angle, he is wearing his multicolor leather jacket, but from the other angle, he is not wearing the jacket.
When 'Ranger X' is pump gun shot so forcefully blasting him up high up against the trailer, afterwards when reverse shot shown lying face down on the floor, there is no resulting (huge!) exit wound on his back.
On the drive to visit the parents, the views out the windows of the car are out of sync and don't make sense. One scene out the rear window makes the car look like it is moving completely sideways. This sequence is obviously a montage, and intended to look this way by the director.
During their make-out session on the couch, as Starlene drops the Polaroid camera, the photographs fall with a clunk, but the camera drops soundlessly.
The trio crosses into Mexico (heading for Acuna) at a land crossing (one guard, small guard shack with a drop arm barrier across the road). All crossings between Del Rio, Texas and Acuna, Mexico have to cross the Rio Grande River at a bridge. In addition, all U.S.-Mexico border crossings have both U.S. and Mexican border patrol guards.