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.
Continuity mistake: During the sequence where Dorothy meets the Scarecrow, Dorothy's pigtails are first short (above her shoulders) and as the song progresses her hair gets longer (below her shoulders), then short, and then long again.
Continuity mistake: During the scene when Dorothy and Scarecrow are fighting with the trees, Scarecrow says "I'll show you how to get apples" and he gets hit by the apples. The very next scene if you quickly look at Dorothy's shoes, you can see that she is wearing black shoes, not her ruby slippers.
Revealing mistake: When the Lion is pulling up the Tin Man with his tail on the way to the castle, you can see a square block inside the Lion's costume to hold the tail on.
Character mistake: While singing the "If I Were King" song, the Lion asks, "What makes the Sphinx the Seventh Wonder?" The Sphinx is not one of the Seven Wonders of the World; its neighbour the Great Pyramid of Giza is. Besides, how would the residents of Oz know about our Seven Wonders of the World?
Continuity mistake: When Scarecrow, Tin Man, and the Cowardly Lion are marching into the Wicked Witch's castle after taking the guards' uniforms, all three of them are shown holding the same types of spears as the guards, so when they go rescue Dorothy out of the locked room and Tin Man chops through the door with his axe, where did the axe come from?
Audio problem: When Dorothy, Scarecrow, and Tinman are in the forest where they find Lion, the first time Dorothy says "Lions and tigers and bears, oh my" her mouth doesn't move at first but when the camera cuts to the next shot her mouth finishes the phrase.
Continuity mistake: Immediately following the Tin Man's dance, Dorothy and the Scarecrow try to keep the Tin Man standing on his feet and clumsily shuffle over to a tree trunk where the Tin Man then sits. During this, the oil can in Dorothy's basket clearly falls out and on to the Yellow Brick Road. Dorothy doesn't pick it up but, in the very next shot, the oil can is back in the basket.
Revealing mistake: Soon after Dorothy slaps the Cowardly Lion, she looks as though she is about to laugh out loud. This happens about 10 seconds or so after the slap. She even tries to put Toto in front of her face to cover up her smirking.
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.
Continuity mistake: In the beginning while Dorothy is still on the farm, she walks along the pig pen fence and then falls in. When Bert Lahr picks her up out of there her dress is perfectly clean.
Continuity mistake: The blue shirted Lollipop Guilder comes from around the fallen house at the beginning of "Come Out Come Out," but he immediately appears emerging from a jet black manhole (with a black lip where the cover goes) in middle of the yellow brick in front of the house.
Continuity mistake: In the scene at the Witch's castle, when the Scarecrow looks up at the candles hanging from the ceiling, the candles are lit, but when he chops the rope with the Tin Man's axe, and the shot is from above, showing the candles fall onto the men, they are not lit, but the next shot shows the candles on top of them and they are lit.
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.]
Continuity mistake: Immediately after Dorothy throws the water onto the witch, there is a long shot of the group as they observe the Wicked Witch beginning to melt. In this shot you can see that the Scarecrow's arm is still on fire, yet nothing is done about it as they all focus on the Witch. In the next full shot of the Scarecrow the fire is completely out.