After the robbery, Doc and Carol's blue car plows through a neighboring porch. The windshield is clearly shattered by one of the broken porch columns. As soon as they are out of town, the blue car is immaculate.
When Carol discovers in the train station that her locker key has been switched, she is holding the key for locker number 090. When she hands it to Doc and we see it shortly after, the locker number on the key is 249.
When Doc finds he can't shoot Rudy after knocking him out in the hotel, he unloads the gun and drops it next him. Later on during the shootout Rudy awakens, grabs his gun, and flips the empty cylinder shut without reloading it. But when he comes out the window to shoot at Doc, his gun is miraculously reloaded.
In the scene where Doc and Carol pull up to Benyons Ranch (4648) the driver's side of the white Ford wagon has 2 different hubcaps. The rear is correct while the front hubcap is a replacement. Later in the scene where Doc is leaving the wagon on top of the parking garage (5703) the replacement hubcap is on the rear while the original is now on the front.
In the final shootout scene with Rudy in the hotel we see Rudy's pistol alternate between the 6 inch barreled .357 Magnum Colt Python he has used throughout the film up to this point and a 6 inch barreled .357 Magnum Colt Trooper. The most obvious difference between the two handguns is the full length rib beneath the barrel. This is noticeably absent on the Colt Trooper. Rudy changes between the guns from shot to shot.
When Doc and Carol flee their shootout with the cops in the small town of Fabens, south of El Paso, they are driving a blue 1968 Chevy, which they then ditch in a lot outside of town to jump on a bus. Subsequently, Carol buys a grey Mercury to continue their trip to El Paso, but as they stop at the drive-in, a news report on the car radio states that police are looking for the grey Mercury, when in fact the last vehicle the cops saw them in was the blue Chevy.
When Doc checks in to his old friend's hotel in El Paso, he asks him to carry his wife's luggage up to their room, but Laughlin declines, saying he is alone and can't leave the desk. This exchange is important to Doc's later realization that a trap is being set for him, since he knows that Laughlin never works alone because he needs someone to keep him from drinking and gambling. That clue would never have dropped in the first place, however, because Doc and Carol have no luggage. She enters the hotel with only the money satchel, and they wouldn't need anyone else to carry that.