Revealing mistake: When the robot releases the balloon with the bomb attached, the wires used to pull the balloon up are visible as the balloon rises through the skylight.
Adventures of Superman (1952)
1 revealing mistake in season 6
Starring: George Reeves, Bill Kennedy, Jack Larson, John Hamilton
The Dog Who Knew Superman - S2-E9
Other mistake: A gangster pushes a dog into a well and the dog can be heard splashing around. The man's wife is furious and several people are unable to retrieve the dog. Superman tunnels into the ground and into the bottom of the well and emerges holding the dog. Both he and the dog are completely dry.
Lois Lane: Eureka.