Continuity mistake: In the scene where the Wicked Witch in Munchkinland goes over to observe the ruby slippers on her dead sister's feet, when she is up close there's not much space between her and the slippers but when she turns around and says "they're gone," she's further away, then she's closer again in the next shot.
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.