Early in the movie when Joe Jarrett is riding away on his horse, there is a white car in the distant background driving from right to left. It comes out from behind the big rock on the left lower corner of the screen.
(at around 1h 20 mins) The movie is set in 1870. Joe Jarrett raises a mug that's marked on the bottom: Wallace-China-Los Angeles-California. According to Potteries of California, the Wallace China Company operated from 1931 to 1964.
When Joe leaves with the money bag, Zack waves to him. Then the big rock on the left side of the screen has a branch in front of it but no shadow at all. Zack lights a cigar and turns around the stagecoach and the big rock is a different one, with shadow and no branch any more.
When Zack shoots Joe on horseback, the landscape behind Joe changes from one shot to another. Joe also falls with no gun in his hands. Next shot, while he rolls on the ground, the gun appears in his right hand.
When Joe goes up to the bar with Zack and asks for brandy, his left hand is around the hat on the counter with his right hand touching his lips. In the subsequent shot both hands are in different positions.
When Chad enters the room to invite Jarrett to lunch Joe grabs his hat and holds it with his hand inside and later as Zach and Joe are heading to the bar Joe stops to grab his hat, remarking Zach would understand why, indicating the gun he carries in it. The camera shows a glimpse of the inside of the hat however and no gun is visible.