During the part where Cooper kicks a fire onto Spangler, burning his jacket, and Cooper then makes his getaway, the two white smoking burn spots on Spangler's jacket change position, size, shape, and color several times from shot to shot.
When Spangler ducks inside the doorway to shoot upstairs at a hiding Cooper, a bullet hits the door frame beside Spangler's head before Cooper even jumps out to shoot back down the stairs at Spangler.
During the gunfight between Spangler's gang and the men defending the town, one of the outlaws is seen twice being shot and falling from his horse.
After Spangler's jacket is burned in two spots at the fire, the burn spots disappear.
Clint has the left sleeve of his shirt torn and his arm ripped open with a baling hook, leaving blood all over him. However, after the fight, when he is sitting in jail, his shirt is ripped, but there is no blood on it and no evidence of a wound.
In the gunfight between Clint and Spangler towards the end of the movie, you can clearly see several of the wooden crates have been spray painted to look older.
During the fight between Clint and the Morrisons there are clearly stunt doubles in part of the fights for Clint and Tom Morrison. Clint's double has a higher hairline and Morrison's hair is a different color blonde. The stunt men don't try to hide their faces either.
When Dan lets Clint out of the cell, they stand in the sheriff's office watching Spangler's gang through the window. The lights in the office are all lit. Anyone in the gang would have seen them quite easily.
When Clint arrives in town he goes into the sheriff's office. From outside, the open door cannot be seen, it is completely in the shadows. However, when there is a close up of Clint entering the office, the door is lit and can be clearly seen.
When Audie Murphy visits Merry Anders in the schoolroom, many of the paintings on the wall behind her are clearly done by 1960s children - a modern house, lamp etc.
When the inside of the church is shown, it shows a copy of the painting "Christ In Gethsemane" by Heinrich Hoffman that was painted in 1890. It is reasonable to assume that this painting was done later than the setting of the movie.
When Ten De Corsia is dragging the woman out of the church, he says they'll be in the "salon" instead of "saloon".
Helen Reed's hairstyle is very 1960s and far too modern for the era.