Revealing mistake: At one point in the movie, dumptrucks are sent in to contain the Mighty Peking Man. You can tell that these, like most of the movie are toys.
Revealing mistake: When the Black Knight kills the King's champion with his axe, you can see the flash and smoke from the charge which blows the armour apart. (01:32:00)
Revealing mistake: In the beginning of the movie when Orca is about to hit the great white shark out of the water the stick that holds the "head" of the great white is visible as it swims by.
Revealing mistake: After Ethel has died and Brenda is crying over her, there is a slight twitch in Ethel's eyes.
Revealing mistake: The supposed deceased dog is breathing as Rack and spider lady examine him.
Revealing mistake: When the bat flies into the bathroom and attacks Suzy you can see a string attached to the fake bat. (01:19:16)
Revealing mistake: At the beginning, when Gail is attacked by a moray eel - we don't see it, but that's the assumption - and she is banged against the sunken wreck of a ship, and then, when she is released, her mouthpiece is not in her mouth and she tries to swim to the top as fast as she can - all of this is done by a stunt double, understandably, we can see this by her hair, which is much thinner and straighter than Jacqueline Bissett's hair.