Cat Ballou (1965)

Revealing mistake: In the scene at the end of the movie where Lee Marvin is running interference during the escape by shooting in the air, horses pulling a beer wagon get startled and run into the street where the wagon turns over. At the beginning of the shot, when the horses begin to bolt, you can see the stuntman driving the wagon. He is sitting low behind the wooden front of the wagon (which has holes cut in it so he can see out).