Other mistake: During the scene where the teens are performing that "kicking" routine in the gymnasium as led by Tommy Djilas, you can see Harold Hill in the back left of the screen, watching. Right before the number ends, a costume prop can be seen coming from the back middle of the dancers and hitting Robert Preston in the face.
Other mistake: In the opening credits, there is one credit that reads "Miss Winter's costumes by Gene Coffin." This is meant to be referring to actress Shelley Winters and the man who provided her costumes, however the apostrophe has been put in the wrong place. Since her full surname is Winters, putting the apostrophe in between the R and S in incorrect. The apostrophe should have gone after the S to spell out "Miss Winters' costumes" and not Winter's. (00:01:35)
Other mistake: While Ransom Stoddard and his wife are on the train back to Washington, the conductor tells them that the train will be able to maintain a speed of 25 mph throughout the journey. The film is set around 1900, by which time trains were regularly topping 65 mph, and the outside scenery is moving faster than it would for something traveling at 25 mph.