Revealing mistake: While the Seaview rocks violently back and forth from the concussion of the under water explosion, the pencils, folders, papers, etc., on the navigation table do not move.
The City Beneath the Sea - S1-E2
Revealing mistake: When Seaview's crew recover the diver's body in the beginning and clear seaweed from his mask, the "dead" man is blinking. (00:01:20)
Revealing mistake: Crane overcomes his guard in a fistfight by hitting him in the face three times. But if you watch closely, it's apparent that not one of his blows connects. He "strikes" the air above the man's head all three times. (00:32:00)
Revealing mistake: One of the saboteurs sneaking into the palace keeps looking up at someone off camera. He's not watching the door he just entered by, so who's he looking at in what should be an empty corner of the room? (00:00:30)
The City Beneath the Sea - S1-E2
Revealing mistake: On the beach, Molina is petite, thin and small-chested in her wetsuit. Underwater, she mysteriously gains several pounds, bigger muscles and a much larger bust, thanks to a poorly-matched stunt double. (00:12:00)
The Wax Men - S3-E24
Revealing mistake: The heavy metal ring securing the hatch that leads from the flying sub to the Seaview control room bounces up and down as Crane enters the control room.
The Wax Men - S3-E24
Revealing mistake: The explosive squib is visible under the uniform of the wax Chip Morton right before Captain Crane shoots him.
Fatal Cargo - S4-E7
Revealing mistake: The seam on the back of the gorilla costume is visible as he attacks the doctor's campsite.