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 Cary Grant walks into the office of the restaurant of his friend the kitchen has the center island to the right of the dishes tray, right of the light, and right of Cary's view to the dishwasher. There is a single row of dark tiles between the island and the cooks in blue. Later when the dishwasher turns and walks toward Cary the center island has been moved to the left. It is left of the dish tray, left of the light and is now showing three rows of dark tiles between the island and the cooks in blue.