Revealing mistake: After the Wicked Witch exits Munchkinland in a puff of red smoke and flames, you can briefly see sulphur marks left on the trapdoor. (00:31:00)
Revealing mistake: When they first meet the Cowardly Lion, he jumps off of a boulder and obviously lands on a trampoline and jumps off.
Revealing mistake: In the scene where the tornado is about to hit and Aunt Em and Uncle Henry are going into the cellar, you can see that some of the trees are blowing around violently, and others aren't moving at all. (00:16:05)