Revealing mistake: Whenever Dorothy, the Scarecrow, the Tin Man, and the Lion go off into the horizon you can see the line on the floor of the sound stage where the fake horizon is and the scene cuts off right as the four get to that point. (00:57:35)
Revealing mistake: As they exit the Tin Man's scene singing, "We're off to see the Wizard", a bird stretching its wings is casting a shadow on the painted sound stage wall slightly to the right of the house, which reveals it to be a fake backdrop instead of an open area. (00:47:45)
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)
Revealing mistake: When Toto reveals the Wizard by pulling the curtain aside, he doesn't use his mouth; the curtain is tied to the dog. Dorothy can even be seen later removing the rope.
Revealing mistake: As the cyclone carries the farmhouse through the air, it is evident that the walls of Dorothy's bedroom are not solid walls, but fabric-covered flats. They flutter and vibrate in the wind. (00:17:50)
Revealing mistake: When Dorothy's bedroom is flying away to Oz, there's wind blowing everywhere and bits of hay falling. However, the hay is coming from inside the room, instead of outside, and the papers on the table next to the window don't blow away nor move an inch.
Revealing mistake: You can see strings attached to the witches monkeys as they fly, and also strings from their bodies to their wings, to give the wings that flapping effect. (01:11:20)
Revealing mistake: When it starts to snow in the poppy field, Dorothy falls asleep, followed by the lion. In many of the later shots, a thin wire attached to the lion is visible the entire time.
Revealing mistake: When the house lands in Oz, it lands with a bump and Dorothy is laying on her bed with Toto. The objects in the room had moved about a lot due to the twister, and wires attached to the end of the bed and the chair, etc. are really quite clear. (00:18:12)
Revealing mistake: After the Wicked Witch throws fire down at them the Scarecrow kicks the stump next to him and it moves several times. The one behind him also moves. (00:46:25)
Revealing mistake: The candles on the chandelier that fall on the guards are not real. If you look closely at them they're operated with gas. (01:24:50)
Revealing mistake: As the Wicked Witch throws the hour glass down at them the stone statue on the left of the monkey is shaking. (01:24:45)
Suggested correction: It's the monkey that is shaking, not the statue.
Revealing mistake: As the Wicked Witch chases the four yelling "Seize them," they run around what looks like a concrete pillar. If you look closely it's covered with cloth that moves as they run by. (01:25:00)
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: After the doorman tells the four they can't go see the wizard he returns back into the palace and you can see his shadow where it shouldn't be against the painted backdrop wall inside the door on the left. (01:07:25)
Revealing mistake: The Scarecrow tells Dorothy to, "just bend the nail down in the back and maybe I'll slip off" but what she actually does is pull a lever. (00:35:40)
Suggested correction: I have the scene up right now, and the nail on the pole just looks like... a huge nail. Then Dorothy bends it, and the Scarecrow falls off. I'm not denying that it's a lever, because it's entirely possible that it is. The thing is, there is nothing in the scene that indicates that it absolutely has to be a lever, and it can't be a nail at all.
Revealing mistake: In the Haunted Forest, when the Tin Man is lifted up then lands back on the ground, at Tin Man's lower back you can see his white T-shirt tucked into his pants. (01:14:10)
Revealing mistake: Throughout the movie, the tin man's costume wrinkles up.
Revealing mistake: The glass in Dorothy's bedroom windows don't break, they are removed from their frames as the scene progresses, and if you look at the reflections in them the windows are made of plastic. (00:18:45)
Revealing mistake: As Dorothy is about to walk into the cyclarama (backdrop) as she leaves MunchkinLand, a Munchkin Soldier on the very far right is too far and becomes transparent with the matted-in flowers. (00:33:40)
