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.
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.
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.
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?
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: 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?
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.
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: During the Lion's song of, "If I only had the nerve." You can see the cameraman's shadow pass over Dorothy, the Lion, and then the log on the side of the road. Watch carefully, it comes back.