Years ago they explained on T.V. about that
blooper. Seems during the filming they were
trying on different lengths of wigs with pig tails on Dorothy and never bothered to correct it afterwards. Thats why you see different lengths
of pig tails. Dumb yes but thats movie making
The reason it was not corrected is because by they hired several new directors and each one changed things about the movie. So, they'd re-shoot a scene and Dorothy's hair would be longer than it was when they'd shot the scene that goes before or after it. For those that don't know: movies are rarely shot in the sequence that they appear on the screen. This slip-up with Dorothy's hair was not fixed because by the time they caught it, they were already WAY over budget and, frankly, they were sick and tired of re-shooting scenes.