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: What appears to be a long hallway in the palace is actually a painted backdrop. When two guards lock Ulysses in the dungeon, they cast a shadow on the backdrop.
Other mistake: When the villan's car is stopped by the traffic jam, the background projection is still moving.