Factual error: Toshiro Mifune's character, Kikuchiyo, travels to the bandits' camp and steals one of their two remaining fuselock muskets. Shortly afterward, the bandits fire twice at the samurai within five seconds with their last firearm. A fuselock takes as much as two minutes to reload, prime, and fire.
Continuity mistake: In the barn dance scene Gideon is doing flips on the sawhorse. His shirt becomes untucked and then retucked between flips.
Revealing mistake: When O'Keefe takes his bride-to-be Dolaroo to Hong Kong to be married, a 1950's truck goes past in the background. Unfortunately, the film is set in 1870.
Factual error: The giant squid's tentacles aren't shaped like ones in the real world are.
Deliberate mistake: After the airplane loses its propeller and descends to lower altitude, the cockpit constantly shakes due to turbulence, and the flight crew complains about it in one scene. However, when the camera switches to the passenger cabin, everything remains perfectly still and serene until the airplane is on its final landing approach near the end of the movie.
Continuity mistake: When Richard Denning and Richard Carlson approach the boat ladder coming back from their first dive in the Black Lagoon, you can see the net hanging underwater just to their right. It already has the huge torn hole that is in it when they hoist it up after the creature is caught for a moment in a later scene.
Factual error: In order to make Polyphem drunk, Ulysses and his men produce 'wine' by treading on grapes. Only thing is: without fermenting the stuff for a few months, all you get is grape juice, which is not even remotely intoxicating.
Continuity mistake: When Marilyn Monroe tries to untie the raft and Robert Mitchum can just catch the rope it the river flows from left to right. Later, when they take off again together, the river flows from right to left.
Continuity mistake: In the scene set during the King Arthur and the Round Table era we see the end of a battle scene. The camera pans in on some dead enemy and we see a man in a helmet face down. The scene then changes but comes straight back to the dead enemy. The man in the helmet has turned away as if to survey the scene before him. Within a second he turns his head back to the original "dead" position. (01:19:30)
Continuity mistake: The two workers are being dismissed by their boss after their 'too zealant' repairs of the streetcar. Shot from behind when he stops them as they walk away, you can see him place his right hand flat on the desk, directly on the wood. Next shot and his hand is in a different position and on the papers. (00:05:30)