If you look very closely at the Wicked Witch that Miss Gulch transformed into while Dorothy looks out her bedroom window during the tornado, you will see a shimmer from her shoes-she's wearing the Ruby Slippers. That means that she is the Wicked Witch of the East, who is soon to be killed when Dorothy's house falls on her.
Margaret Hamilton has never been credited for playing this role (which is practically a cameo). This shimmer from her shoes is even more obvious when watching a better-quality copy of the film, such as the 1989 50th anniversary laserdisc version or the 1999 60th anniversary Warner Bros. DVD restored version.
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