When Tom frees the colt, his shirt is covered in wet mud. A second later it has all dried up.
In quite a few (European) countries, the local title of this film translates to "Duel in Missouri". However, the film takes place in Montana, in the Missouri River Breaks area.
Tom tells Woodruff to hand him the Birdshead Colt he has stuffed in his shirt, but you can clearly see that the Colt has the standard grip, and is not the Birdshead grip at all.
When Tom and Jane mount the same horse, one in front each other, her modern white underwear appears for a while.
When robbing the train, Tom Logan tries passing himself off as Jesse James who died in 1882 indicating the timeline is the 1880s. Later Calvin is singing When the Roll is Called Up Yonder which was written by James Black in 1893.
Cary gives a boy a dollar, and tells him to get some good whiskey and keep what's left. The boy returns with a glass of whiskey, but also returns the change.