Continuity mistake: When the Munchkins come out onto the Yellow Brick Road, one of them comes out of a sewer hatch on the yellow bricks. It's located on the ground in front of the first small white cottage with the thatched roof, right behind Dorothy's house. Throughout this scene, the dark hatch is visible whenever we see that specific location, such as when the Wicked Witch of the West asks, "Who killed my sister? Who killed the Witch of the East? Was it you?" but as Dorothy begins to "follow the yellow brick road", there is an overhead shot followed by a front shot and the dark hatch has disappeared.
Continuity mistake: When the four are at the end of the hallway to see the Wizard, Toto is near the lion's feet. As it switches shots, he is behind The Tin Man.
Continuity mistake: Throughout the film, if you look at the Wicked Witch's nose, you can see that in some scenes the prosthetic nose is barely longer than Margaret Hamilton's real nose. In other scenes, the tip of her fake nose hooks almost all the way down to her mouth.