When Dorothy firsts meets the Scarecrow, her pigtails change from short and curly to long and straight several times.
After escaping from Miss Gulch's basket, Toto returns to Dorothy's room by jumping through the rectangular opening where there should be a windowpane. Soon after, the twister blows the window out (that wasn't there when Toto came back) and knocks Dorothy unconscious.
When the group is asking the guard if they can see the Wizard, his mustache suddenly goes from being pointed upward to downward between shots. This is the result of a deleted scene.
During the twister scenes, as Aunt Em is calling for Dorothy, there are some objects in the background (including a tree that isn't blowing) that disappear when Dorothy arrives and tries to get Aunt Em to let her in the storm cellar.
As Dorothy and Toto run down the dirt road toward home at the beginning of the film, Dorothy bends down to pick Toto up and you can see a grease spot on the front of her dress, but by the time they reach home, it has disappeared.
The Wizard tells the Scarecrow, "Why, anybody can have a brain. That's a very mediocre commodity! Every pusillanimous creature that crawls on the earth or slinks through slimy seas has a brain!"
He's right about the earth-crawlers, but the sea contains many creatures without brains, including jellyfish, corals, Portuguese man o' war, starfish, clams, oysters, sea cucumbers, sea urchins, sea sponges, sea lilies, sea anemones, and sea squirts.
The Wizard of Oz original motion picture soundtrack incorrectly labels the twister as a cyclone. While both cyclones and tornadoes involve rotating winds, the key difference lies in their scale, formation, and location. Cyclones are large, long-lived storms that form over warm ocean waters, while tornadoes are smaller, shorter-lived, and typically form over land.
The Wizard states that he is "an old Kansas man" yet his balloon is registered to Omaha, Nebraska. Midwestern carnival performers often travel between two or three states on a regular route.
Glinda tells Dorothy never to let the ruby slippers off her feet, but apparently the slippers remain stuck to Dorothy's feet after the Witch of the West claims they can't come off while Dorothy lives. Obviously this kind of speech is part of Glinda's effervescent character.
Tin doesn't rust. However, the movie takes place in a fantasy world, where it can do so if the filmmakers say it can.
When the group enters the castle disguised as Winkies, the Tin Man is carrying a spear. His axe reappears seemingly from nowhere to smash the door open to free Dorothy. The Winkie costume was very bulky with more than sufficient room to conceal the axe.
Houses in the Mid-West (Tornado Alley) are built with storm cellars that you can enter from inside the house, not outside the house. This prevents being injured or killed by an approaching tornado. Although this is more common now, many homes in the mid-west are older homes that do not have storm cellar entries inside the house. You still have to go outside to enter the shelter, so this is not an unreasonable scene to show in the movie.
When the Wicked Witch of the West disappears from Munchkinland in a cloud of smoke and fire, the smoke appears before she reaches her mark.
The purple "Horse of a Different Color" can be seen licking off his purple dye, which was made of grape-flavored gelatin powder.
During the apple tree sequence, Dorothy's slippers momentarily disappear (she is wearing black patent leather shoes). This happens when the tree throws an apple that makes Scarecrow fall over. The black shoes are visible bottom right, just as Scarecrow lands on his bottom.
The Wicked Witch of the West appears in Munchkinland in an explosion of smoke and fire, but the Munchkins start running and screaming before it.
After the scarecrow was re-stuffed because of his body being severed by the monkeys, he stands up, and trips slightly in the hole that was meant to conceal the lower half of his body.
After saying, "Well, my little pretty, I can cause accidents too", the Witch says something else which cannot be heard. The script backs up this claim. The additional dialog was, "And this is how I do it!" Margaret Hamilton's lips can be seen forming the words "And this" without sound before the shot cuts away.
When Dorothy uses the knocker on one of the Emerald City doors, four knocks are heard although she only hits the door three times. The first one is heard while she is drawing the knocker back.
The Mayor and his agents' voices get mismatched in at least two places. During the congratulatory music, a "lawyer Munchkin" interrupts the festivities and demands legal verification of the Witch's death. When they exchange words saying, "From now on you'll be history! You'll be hist - you'll be hist - you'll be history!", the "lawyer" Munchkin has a completely different voice, as though from a different actor. Furthermore, when the Mayor says that last "history!", we hear the "lawyer" Munchkin's voice instead.
The real wizard makes the fake wizard's voice by talking directly into the microphone, but when the fake wizard says "Pay no attention to that man behind the curtain!" the real wizard is pulling back the curtain, pointed away from the microphone and not talking at all.
When the group chants, "Lions and tigers and bears!". just before encountering the Cowardly Lion, and it is Dorothy's turn to say the first, "Oh my!", her mouth does not move.
In the close-up of the Palace Guard crying, tears can be seen dripping from a device attached in his false eyebrow.
A flesh-colored hand is visible in the upper left-hand corner of the frame in the closeup of the Witch being splashed in the face with water. The Scarecrow is wearing dark burlap gloves, the Winkies are wearing black gloves, and Dorothy is holding the bucket, so the only explanation is that the hand belongs to a crew member throwing water on the Witch.
The cameraman's shadow is visible as it pans across the Sleepyhead Munchkins' nest in Munchkinland.
When we first see Munchkinland, you can see the shadow of the camera pass over in one of the establishing shots.
Studio lights are reflected in Dorothy's bedroom window as it blows out of its frame and knocks her unconscious.
The twister seems to teleport between shots. this can be seen when it's behind the farmhouse but then jumps miles away and changes position. This can also be seen when Dorothy is entering through the gate and it's about 50 meters away from the farmhouse but when we see the adults closing the storm cellar door it's now back where it was when Aunt Em was calling for Dorothy.
When the Witch sends out the flying monkeys, she says, "I've sent a little insect on ahead to take the fight out of them." Those "insects" are Jitterbugs. The Jitterbug scene was filmed, but later cut. The line, however, was not removed.
Miss Gulch is not brought up in the final scene, leaving her threat to have Toto put down unresolved.
When Dorothy is balancing on the pigpen fence, you can see a boom shadow pass over the scene.
After the Wizard gives the Scarecrow his diploma, he says, "The sum of the square roots of any 2 sides of an isosceles triangle is equal to the square root of the remaining side." This is a misstatement of the Pythagorean Theorem, which is, in fact, about right triangles and not isosceles ones. However, this statement is not true about any triangle, and so it is completely wrong.
Dorothy smiles and tries not to laugh after she smacks the Lion and he asks, "Is my nose bleeding?" Judy Garland buries her face in Toto to conceal this.
During the "If I Were King of the Forest" song, the Lion asks, "What makes the Sphinx the seventh wonder?" The Great Pyramid, not the Sphinx, is one of the seven wonders of the world.
In the "King of the Forest" scene, Dorothy trips on the emerald carpet on her way back up the stairs.