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.
Other mistake: When Travis shoots Yeller, his gun (a .50 Cal black powder rifle) doesn't kick at all. Ordinarily a gun of this type kicks hard enough to leave a bruise.