When Cody stays behind to shoot the oncoming riders, just as the riders come around from behind a mountain and Cody raises his gun, you can see one rider throwing his arms up and falling off of the horse. One second later, Cody shoots, and the next view is a second shot of that same rider raising his arms and falling off his horse.
During the final shootout, when the view is the back of the riders shooting toward the canyon, a person is seen slowly walking over the mountain toward the upper right portion of the scene.