In the scene where Bonnie and Clyde come back for Buck, a close-up of Clyde reveals that the Ford Model A has a modern dash with a column shift overdrive automatic transmission; which would have replaced the original 3 speed manual which the car would have been originally equipped with.
At 2:24:05 in a shot of Frank Hamer (William Hurt), there is a cable TV line visible behind him which didn't exist in the early 1930s.