Other mistake: When Kirk Douglas 'appears' to pull a knife from his collar and throw it into Lee Van Cleef's chest, you can see Kirk pick the knife up from a trough in the front edge of the bar. The knife is already in his hand when he reaches to pull the knife from his collar.
Deliberate mistake: At the beginning of the movie Mr. Coates tells his wife that he will be gone "all of three months, maybe four." The first time we see Bud and Elizabeth Searcy she tells Travis that her dog is pregnant and that Old Yeller is the father. We then see her dog that does not look very pregnant. The next time we see Elizabeth she presents Travis with one of the pups. The problem is that the pup that was supposedly just born is about 4 months old.
Continuity mistake: In the scene where Glen Ford shoots the stagecoach driver dead, the body falls to the ground. Glen Ford gives the order to put the "dead" man on the saddle of a horse. When the men lift him up, the "dead" man puts his right hand up onto the shoulder of one of the men.
Factual error: When Brennan rides into town, behind the stage coach in the back street there is a pick up truck parked.