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: 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.