Continuity mistake: As Shirley Jones is singing "In love with Me" at one point, she's sitting down, and is slouching back. The shot changes to the side, and she's sitting up straight, and is much higher against the counter than she was. Her hands have changed position as well, she's no longer holding onto the cup she had in her left hand.
Revealing mistake: During "Marian the Librarian," Marian is seen doing a little dance on the table before being hushed by the people in the library. Like the incident with Winthrop in "Gary, Indiana," you can see that she did not actually do this dance. Tap sounds were added in later to make it seem like she did.
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Trivia: Shirley Jones was pregnant during the movie. She wore a special corset to conceal this. Director Morton DaCosta did not want anyone else involved in the movie to be aware of her pregnancy. He was so successful, that it was not until the kissing scene on the bridge that Robert Preston (Harold Hill) found out, when he was shocked to feel Jones' baby kicking him in the stomach.