When Dorothy meets scarecrow, her pigtails are first short (above her shoulder) as the song progresses, her hair gets longer (below her shoulders) then short, then long again. If you count, her hair changes length five times in that one scene.
In the original cut of the film, the Scarecrow has a long extended dance number at the end of "If I Only Had a Brain". For time constraints the dance number was cut and some bits of the scene were reshot to create a smooth abridgement of the dance sequence. During the retakes Dorothy's braids were somewhat longer as a result of a mistake by the make-up team. Judy Garland's hair did not grow; the braids were extensions throughout the entire film.