Singin' in the Rain (1952)

Other mistake: In Gene Kelly's 'Singing in the Rain' sequence, the taps he makes do not match up with the actual movements of his feet - they are much more complicated than the steps he makes. Since one can't tap dance in wet loafers, Kelly had to do the dance in the studio, and the taps were added into the picture. He apparently got a little overzealous.

Don Lockwood: What's your lofty mission in life that lets you sneer at my humble profession?
Kathy Selden: I'm an actress.
Don Lockwood: What?
Kathy Selden: On the stage.
Don Lockwood: Oh, on the stage. Well, I'd like to see you act. What are you in right now? I could brush up on my English, or bring along an interpreter. That is if they'd let in a movie actor.
Kathy Selden: I'm not in a play right now, but I will be. I'm going to New York...
Don Lockwood: Oh, you're going to New York! And then someday, we'll all hear of you, won't we? Kathy Selden as Juliet, as Lady Macbeth, as King Lear! You'll have to wear a beard for that one, of course.

Trivia: Donald O'Connor filmed the 'Make 'em Laugh' number in one day, over several takes. By the end of that one sequence, he was so bruised and battered from the repeated unpadded pratfalls, he could barely move. He was unable to work for several days after.