Revealing mistake: In "Wonderful, Wonderful Day" there are live birds flying on the set, but they were confused by the backdrop and several crash into it. (The director acknowledges this in a documentary.)
Revealing mistake: In the scene where Milly is singing Wonderful Day and the bird come flying past her, their shadow falls on the "hills" behind them.
Revealing mistake: During winter at the cabin, several of the kidnapped girls are doing chores. As they are about to go back inside the cabin, the long icicles hanging on the porch eaves can be seen swinging loosely when they should be frozen in place.
Answer: Believe it or not, there were actually two original versions of Seven Brides for Seven Brothers filmed simultaneously in the early 1950s. One version was filmed in "CinemaScope" format and the other version was filmed in "Flat Screen" format. They did this in order to accommodate theatres that couldn't run CinemaScope. They would film a scene or scenes in one format in the morning, then film the same scene or scenes in the other format in the afternoon, using the same actors, costumes, sets, et cetera. Naturally, the camera angles, camera movements, performances and even sound recordings are noticeably different from one version to the other. The non-widescreen one wasn't used, but was released on DVD, and there's a chance TV stations might have used it before widescreen became more prevalent.