The movie opens with an Atcheson Topeka and Santa Fe train making its first run to Dodge City in 1866. However, Dodge City wasn't founded until 1871, and the ATSF line to Dodge City wasn't completed until 1872.
The fast passenger steam locomotives of the era had only 2 large driving wheels each side. The slow freight locomotive in the film has 4 driving wheels each side. The director has had 2 of these 4 wheels painted white to simulate a passenger locomotive. The locomotive in the film is a more modern locomotive and so it is able to speed along even though it is a freight locomotive.
During the opening, there is a race between the stage and the train. The location of the stage changes between shots from being even to the passenger car (as viewed by the passengers) to being even with the locomotive (as viewed from the stage).
During the scene where the angry townspeople gather en masse outside the jail after Yancey is arrested, just above the rooflines of the buildings is seen a group of tall palm trees - certainly not native to Dodge City, Kansas, but relatively plentiful outside the studio sound lot in California.
During a gunfight scene a wagon pulls into field of fire. The passengers are obviously dummies. All sit perfectly still and upright, however one of their heads wobbles about quite noticeably. It was probably decided that it would be safer to use dummies, but it's just blatantly obvious.