When you change directors four times - including bringing one more in after the main one's finished because you need someone to tidy up a few things and film a little bit of extra material - you're bound to have a few little inconsistencies. The job of a good continuity editor is to pick up on these mistakes and arrange for them to be corrected before the film is released. We can only assume that 'The Wizard of Oz' did not have a good continuity editor.
For a start, for all the attention they were paying to Judy Garland's weight, they were paying none to her hair. Garland didn't get a haircut at any point whilst working on the film (probably because she was too drugged up or didn't have the time), so the length of Dorothy's hair changes considerably from shot to shot. Characters have props in some scenes that either change or disappear in the next, and actors switch positions between shots. It's a mess.