Revealing mistake: In the scene where the Lion sings his "If I Were King" song just before visiting the Wizard, if you watch closely above his moving tail, there is a string holding up and moving around the Lion's tail. (01:04:15)
Visible crew/equipment: When Dorothy lands in Oz and exits the house to Munchkinland, the camera pans around the land. When it slowly pans over a large bed of tall flowers, you can clearly see a very large shadow passing over them. Obviously the shadow of camera equipment or a camera crane. (00:19:10)Hamster
Continuity mistake: There are two actresses playing Dorothy (and two dogs playing Toto) as she opens the door to Oz. The first is a stand-in wearing a dull colored dress in order to make it look black and white. After she opens the door and moves out of the way, Judy Garland is who we actually see walking through. Look at the different pleats on the two dresses. (00:19:25)????