Revealing mistake: In the scene just before the final fight scene, when Burt Lancaster is in the balloon and is approaching his 18th century sailing ship, the scene gradually changes to a view of the side of the ship. Just then and for a few seconds you can see a white cruise ship on the horizon in the background, just above the deck and to the left of the forward mast.
Revealing mistake: At the first premiere (at Grauman's Chinese Theatre), look closely at the audience. Because at the end of the movie, and several months later at the premiere of The Dancing Caviler, it is the same audience! (Same clothes and even the same seats!)
Revealing mistake: When the Prince and Princess are catapulted over the wall of the Giant's castle, you can see their shadows on the "sky" above the wall; the sky is a studio wall painted blue.
Revealing mistake: In the opening scene the Club's dancers are in full-swing, including the tall, lanky gentleman from Lautrec's famous Moulin Rouge poster, the one with the very angular, extended nose and chin. In several closeups it's very plain that the chin is a makeup appliance: the edge of the appliance isn't blended very well with the actor's real skin.