The Hostage - S6-E2
Other mistake: A gang kidnaps Ben Cartwright and holds him for ransom. He is chained to a post in an abandoned mine. When the men walk around inside the mine, the sound of their footsteps make it obvious that the floor of the mine is actually plywood covered with a layer of dirt. Additionally, when Ben finishes eating stew that was brought to him and sets the container down, it makes a loud "clunk."