In the script, the
landing party crewmembers who were killed were supposed to have been
miraculously revived at the end of the episode, but this scene was
cut, however, it would explain why Mr. Leslie reappears in future
Why is Spock trying to prevent the entry of the gaseous creature using his bare hands - this doesn't seem very logical.
When Spock opens a channel for Kirk to talk to the bridge at the end, the activation light on the intercom doesn't go on.
Eddie Paskey, who had
played the character of Lieutenant Leslie in numerous episodes,
meets his demise at the hands of the cloud vampire. However, the
character still reappears in episodes after this one.
Dr. McCoy's medical tunic had an engineering patch on it instead of the sciences patch is should have.
When Kirk and Garrovick are fighting, Kirk throws the ensign up against a heavy rock wall. The wall noticeably moves from the impact.