Revealing mistake: Every time the octopus kills someone, it is painfully obvious that the octopus is fake, as it never moves. The actors are only lying on top of it with a couple of tentacles wrapped around them, and thrashing about to make it appear as thought they are being killed. It never really works.
Revealing mistake: When Dr. Vornoff has been subject to his own machine, and has gained "the strength of 20 men", you can easily see the platform shoes worn by Bela Lugosi's stunt double to make him bigger than Tor Johnson. Dr. Vornoff did not wear these shoes when he was strapped to the table, they only appear after he gets up.