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.
Audio problem: When they are filming the "Duelling Cavalier" with sound for the first time, when the operator says that "She's gotta talk into the mike. I can't pick it up." his lips aren't moving at all. (00:49:33)