Corrected entry: Near the end, Clint Eastwood is sneaking up on a house at night. The old guy (Ed Begley Sr) shoots at Eastwood repeatedly until his gun is empty as Eastwood dodges from tree to tree. Just before running directly to the house, there is a strange black circle that completely covers Eastwood for a few seconds. It is very obvious and no reason why it should be there.
Corrected entry: 6 men were to be hanged. The hangman only pulled 4 trap rings.
Correction: When they hang the 6 there are 2 hangmen. Both pull rings at the same time.
Corrected entry: After Clint Eastwood is cut down from the tree by the Marshall and put in the jail wagon, his right eye is black and blue and swollen shut. An hour later when they pull into town the eye is completely healed.
Correction: Some of the Marshals journeys lasted many weeks. Half way through the film Clint meets another Marshal on his way out of the court house and he says something along the lines of "see you in 7 or 8 weeks, give or take a month". It would be reasonable to accept that Clint was in the wagon for a few weeks. It just wasn't shown.
Correction: It sounds like you're describing a cigarette burn. This is an indicator to the projectionist that a reel change is upcoming, and not a mistake.