Before going to town to kill the sheriff all but 1 bullet was removed from Lucas' gun, however when they get to town he shoots his rifle twice.
Lucas shoots his rifle once and then grabs the rifle from a fallen henchman to take his second shot.
Lucas shoots his rifle once and then grabs the rifle from a fallen henchman to take his second shot.
Near the end when shooting is over and while Lucas and Mark are hugging, a man falls off his horse in the background.
Man falling off horse is NOT a mistake. He's one of the outlaws who was just shot.
The man falling off his horse at the end WAS a mistake. The two villains who were shot, fell to the ground. One villain was knocked off his horse by Lucas. The surviving villain (Don Drysdale) surrendered.