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.
Audio problem: When Donald O'Connor walks onto the keys of his piano at the beginning of his Make 'Em Laugh Sequence, the sound that we hear is a chord, but in reality we would hear a much more dissonant combination of notes, because several notes adjacent to each other would be played at the same time.
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.