During the scene where the family is shot in the backseat of the Chrysler, two double barrel shotguns are used. There are five slug holes in the windshield and even more shots are heard. (The guns were not reloaded and the maximum number of shots available would be four)
The Ford Maverick that Tison and the rest hijack from the family (before killing them), has a set of cheap plastic aftermarket hubcaps, an item that was not around until the late 80's/early 90's at the earliest.
When Gary is shown wandering the desert by himself, there are multiple boot prints from various other types of boots (with tread) other than the ones he's wearing (smooth soled cowboy boots, clearly shown).
People living in Arizona in the 1970s did not speak with the southern accents as portrayed in this film. So almost all the accents were inaccurate. Further, state policemen did not wear cowboy hats. (If you notice there were no cowboy hats actual pictures at the end).
During the manhunt for Tison (1978), modern Ford Crown Victorias (2nd gen, 1998-2012) are shown racing to capture Tison.
The Saguaro cactus is native to the Sonoran Desert in southern Arizona.