Revealing mistake: When the Wicked Witch scares the Munchkins in Munchkinland, where Dorothy lands, she disappears into a cloud of smoke she creates. But you can see her sneak down into a trap door below. [As a sidenote to this entry, Margaret Hamilton was hospitalized for severe burns after a take of this shot (not the final one used) when there was a glitch in the timing of the explosion.]
Revealing mistake: When the Wizard is getting ready to take off in the balloon, while most people watch Dorothy climb out of the basket and go after Toto, the Tin Man is unravelling the thing holding the hot air balloon down, then he "accidentally" lets go.
Revealing mistake: If you listen closely, during the singing of "We're Off To See The Wizard" by Dorothy, the Scarecrow and the Tin Man, you can recognise Buddy Ebsen's gravelly voice. He was the original Tin Man, but had a reaction to the makeup. They had already recorded the songs and it was too expensive to re-do it. (This film was WAY over budget and they couldn't waste time and money re-recording songs)
Revealing mistake: When everyone is still in the poppy field, after the snowstorm, Dorothy says "Look. Emerald City is closer and prettier than ever." Look behind the Lion just as Dorothy says the word "City" and you'll see that the wire attached to his tail brushes over the top of some of the poppies, knocking the snow off.
Revealing mistake: In the scene when the Wicked Witch of the West bends down to retrieve the ruby shoes from the feet of her sister sticking out underneath Dorothy's house, there is one quick shot in the middle of the scene that shows her in front of a screen with a picture (film) of the house on it. Maybe that shot was filmed after the original set was unavailable? That would seem to be the only possible and reasonable explanation why such a shot would be necessary.
Revealing mistake: Soon after Dorothy, the Tin Man and the Scarecrow meet the Lion for the first time, they dance along the Yellow Brick Road, singing. As they disappear at the back of the set, their shadows can be seen on the backdrop, equally covering the trees and everything in between. DVD showing.
Revealing mistake: In the scene where Dorothy, Tin man, Lion and Scarecrow are in the haunted forest and are discussing whether or not to believe in spooks, Tin Man is lifted up into the air and then comes crashing to the ground. You can see his pants wrinkle up as he's trying to get up. Since when does tin wrinkle like that?
Revealing mistake: As the Tin Man busts down the Witch's door with his ax if you look closely you'll see a slight edit in the film right before he takes his last swing. What they did was unlock the door before the last swing so it would open. You can see the dead bolt retracted as the door swings open.
Revealing mistake: When the doorman initially refuses to let Dorothy and friends into the Emerald City, they protest, saying the Good Witch of the North sent her; they show him the ruby slippers as proof. As the camera pans down to the slippers, look at the yellow brick road Dorothy is standing on - it is flat and looks as though it was simply painted.