When Roy Earle leaves Indiana, he's driving a 1937 Plymouth coupe. As he's approaching California, the car is a 1938 (similar to the '37 but shorter, wider grille). Later, it changes back to the '37, then to the '38 again.
When Roy Earle, traveling under an alias, first meets Pa Goodhue at the gas station in the desert, he only introduces himself as "Collins". However, when they meet for the second time after the car accident in Tropic Springs, Pa immediately greets him as "Roy," even though Earle had never offered a first name.
Near the end of the picture, news reports announce that "Mad Dog" Earle is traveling with a girl named Marie and a dog named Pard. How does anyone know about the dog? The only two people who saw Pard in the car with Earle and Marie are dead and Mendoza had no chance to see the dog in the car, or even to get its name from his one visit to the cabin.
When Roy Earle stops at the gas station in the desert he asks the attendant for gas and water, the attendant immediately grabs the water-hose and removes the radiator cap with his bare hands, despite the fact that the engine and cap should have been hot.