After the line, "Well, my little pretty, I can cause accidents too," the Witch is about to deliver further dialog, which we do not hear. The script backs up this claim. The full dialog was: "Well, my little pretty, I can cause accidents, too - and this is how I do it!"
In at least two shots, where the violin players in red and white are visible, as Dorothy is leaving Munchkin Land, the sounds of the violins in the accompanying music do not match the actors' hand movements.
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.
The Mayor and his agents' voices get mismatched in at least two places. During the congratulatory music, one 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!", that "lawyer" Munchkin has a completely different voice, as though from a different actor. Furthermore, when the Mayor says that last "history!", we hear not his voice, but that of the "lawyer" Munchkin.
After Scarecrow is given an honorary 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.
When Dorothy visits Professor Marvel, he takes the picture of her and Aunty Em and puts it under his left thigh while looking into the crystal. As Dorothy is leaving the wagon to return home, he holds the photo up but doesn't give it back to her. Even after he visits at the end of the movie, he still never returns the photo.
The Munchkin coroner sings that he has "thoroughly examined" the Eastern Witch and declares that "she's not only merely dead, she's really most sincerely dead." A few scenes later you can see the house completely covering the Witch with only her feet sticking out. How could she have been "throughly examined"?
After the Tin Man sings "If I Only Had A Heart" and starts his dance in the road, the scene shows Dorothy and the Scarecrow watching him dance; Dorothy's right arm is crooked like she is carrying the basket but the basket is not on her arm. The next scene has her holding the basket on her arm.
When Dorothy and Scarecrow first meet the Tin Man, Dorothy has 2 red apples in her hands. She drops the one in the left hand and switches the apple that is in her right hand over to her left so she just has one apple. While helping Tin Man move his arm, she then drops the other apple. The apple then reappears soon after in a subsequent shot.
After escaping from Miss Gulch's basket, Toto returns to Dorothy's room in her Kansas house by jumping through the rectangular opening where there should be a window pane. Soon after, the twister blows out the window (that wasn't there when Toto came back) and knocks Dorothy unconscious.
At the beginning of the film as Dorothy and Toto run down the dirt road toward the farmhouse, Dorothy bends down to pick Toto up and you can see a grease spot on the front of Dorothy's apron, but by the time they reach the farm, the spot has disappeared.
When the four travelers confront the fraudulent Wizard, the Scarecrow is standing second from the left, between the Tin Man and Dorothy, up until the line, "And Scarecrow's brain?" In the very next shot, however, when the Wizard replies, "Why, anybody can have a brain," the Scarecrow is suddenly standing first on the right, next to the Cowardly Lion.
When Glinda is leaving Munchkinland, all three members of the Lollipop guild and a female munchkin (among others) run towards her pink bubble and shout their goodbyes to her. Those same characters are still standing behind Dorothy in the very next shot as she says "People come and go so quickly here."
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.
When the group gets to Emerald City, the doorman looks downward at the characters when they are in the whole shot. When the shot switches to a close up of just the doorman's face, he looks straight ahead as if Dorothy and the others are eye-to-eye in front of the door's opening.
When the group is asking the Palace Guard if they can see the Wizard, his mustache suddenly goes from being pointed upward to being pointed downward between shots. This is the result of a deleted scene, "the changing of the guard."
The amount of straw hanging out of the Scarecrow's shirt constantly changes. Also, when the Wicked Witch vanishes off the roof of the cabin, the straw hanging from the shirt changes from right to left and back again.
In Munchkinland, when the Wicked Witch is making her way toward the ruby slippers, a mild shadow is cast upon the side of the house. In the next shot that shows the slippers disappearing, the shadow is gone. The shadow then reappears in the shot after that where the which cries, "They're gone!"
One shot shows the ruby slippers disappearing off the feet
of the Wicked Witch of the East. Immediately in the next shot, the Wicked Witch appears, apparently superimposed over a screen backdrop of that scene.
When the flying monkey grabs Dorothy, the sky of flying monkeys shows Dorothy being carried. Behind her, a flying monkey carries Toto. In the next shot on the ground, a monkey then grabs Toto to carry him off.
After the Lion finishes "If I Were King of the Forest", he says, "You can say that again!... Huh?" the close-up shot before the gatekeeper appears shows the lion has his right arm down. In the next shot the gatekeeper appears ("The Wizard says 'go away!'") and the Lion now has his right arm up.
After Dorothy, Scarecrow and Tin man are confronted by the witch, and she leaves, Dorothy is cuddling Toto, and Tin Man says "beehive... baahhh!" Dorothy is then rubbing the shoulders of Scarecrow and Tin Man, saying they are the best friends anyone ever had. But when did she put Toto down?
When Dorothy and the Scarecrow use the oil can on the Tin Woodman, there are vines around the Tin Woodman's left arm and left knee. The Scarecrow briskly removes the vine around the arm, but not the vine around the knee. When the Tin Woodman moves to the Yellow Brick Road to dance, he has no vines on him.
After the witch drops Toto into the lidded basket, a flying monkey takes the basket's handle in his left hand, moves behind Dorothy, and turns around, so the basket is to the right of his body. As she approaches him, the shot changes and the basket is on the left, on stone architectural feature at table height, having instantly moved several feet.
After the Scarecrow, the Tinman and the Lion rescue Dorothy, they run downstairs. As the door slams shut, the order is Scarecrow, Dorothy, Tinman, and Cowardly Lion. In the next shot, Dorothy and Tinman have switched positions.
When the Witch of the West goes to check on the Ruby Slippers, she walks right up to the remains and clearly sets the straw end of her broom against the side of the house, partially covering the feet. In the next two shots showing the slippers and remains disappearing and the Witch's reaction to them, there is no sign of the broom and it appears that she is several feet away.
The Scarecrow is still on fire after Dorothy has thrown the bucket of water. In the first long shot immediately after the closeup of the Witch getting hit in the face; a small flame is visible on the Scarecrow's arm, which disappears in the next shot a moment later.
When Dorothy is in her bedroom at the beginning of the movie the windows are closed with the curtains on them. The twister blows 'both' windows out and the one hits her on the head causing her to pass out. In all the next shots of her in the bedroom, the right hand window with the curtain is back in the shot. It should not be there as it was blown out.
When Dorothy first meets the Mayor of the Munchkin City, his large watch reads 5:00. When the Wicked Witch leaves, it reads 7:15, even though the scene is only a few minutes long and happens in "real time".
After Dorothy is given the Lollipop from the Lollipop Guild and taken over to Glenda before the witch appears, she stands in front of Glenda, the Munchkins scream before the smoke appears. At one point when the witch of the west starts talking to Dorothy you see Dorothy holding the flowers and Lollipop, shortly afterwards you don't see flowers or Lollipop, then you see the Flowers but no Lollipop. You also see Dorothy's hair change from long and semi-straight to short and curly on the end.
After the witch throws her fireball from atop the cottage, the Tin Man extinguishes the flames on the ground, and the witch disappears in a cloud of smoke. The shot of Dorothy, Tin Man and Scarecrow is flopped (and soft on focus), and the screen direction they are looking is wrong. This was done to match the positions of the Tin Man and the Scarecrow in their next full unflipped shot.
A flesh-colored hand is visible in the upper left-hand corner of the frame in the closeup of the Witch being splashed 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.
At the beginning of the film as Dorothy is running down the road and stops to talk to Toto, it is established that the nearest dwelling is at least a mile down the road; but in the next shot a split second later she is running into the farmyard.
There is no stage hand on set at the end of the Tinman sequence, nor was anyone hanged, nor did they fall out of a tree. The shadowy figure at the back of the set as Dorothy, Scarecrow and Tinman set off down the Yellow Brick Road is a large bird stretching its wings.
When Dorothy awakens back home in Kansas and all is seemingly well again, nobody seems concerned by the fact that, although the Wicked Witch of the West may be no more, Miss Gulch is presumably alive and well, and still in possession of a valid sheriff's order allowing her to have Toto "destroyed."
After being joined by the Tin Man, as the three start back down the Yellow Brick Road, the negative is flipped. Refer to orientation of buttons on Scarecrow's shirt, rivets on Tin Man's torso, and orientation of Tin Man's funnel cap. This was probably done to compensate for continuity error in characters' positioning. (Fixed in 2009 remastered Blu-Ray edition.)
When the Witch sees the ruby slippers under the house and approaches Dorothy and Glinda, you can briefly see that her legs are white, not green. This same mistake can be seen when she says, "Ring around the rosy, a pocket full of spears..."
Dorothy apparently almost trips on a wire or rope when she walks down from the hill of poppies arm in arm with her companions. You can see her mouth open in surprise as she almost falls forward, but is held up by the other actors. To the right of the actors, some of the poppies on the hill are briefly bent over by the rope or wire as it is dragged over them.
After The Wicked Witch of the West is looking in her crystal ball and talking to Nikko, we can see windows behind her showing a scene outside. As the witch gets on her broomstick she knocks one of the windows and it is shown to be a sheet with the window painted on. The sheet wobbles.
When the Wicked Witch of the West throws fire at the Scarecrow, the Tin Man uses his funnel (hat) to put the fire out. He then replaces the funnel back on his head, but the shot is a mirror image (his hat and buttons are reversed). In the next shot it is back to normal. This may have been to compensate for a deleted scene.
During the "Horse of a Different Color" scene, you can tell by the height, shape, and other physical features (also by the way the yellow horse trots), that although in the movie it's supposed to be 1 horse changing color, that it is 3 different horses.
When the house finally lands, the furniture and floors are level, indicating that it landed flat and intact. Yet when we see the exterior after Dorothy has entered Munchkinland, the right end of the house's foundation has nearly broken in half over The Wicked Witch of the East, and the siding has sustained major damage.
When Dorothy and her companions are trying to escape from the witch in her castle, one of them bumps up against the "stone" structure, indenting it, and revealing that it is really a canvas flat. (It is very subtle and hard to see.)
As the Lion is singing, "If I Were the King of the Forest," and his tail is flopping from side to side, a string cam be seen controlling his tail. Just before the Scarecrow and Tinman put on the Lion's cape, you can see the Tinman remove the wire with a quick yank.
When Professor Marvel has Dorothy close her eyes "... to be better in tune with the infinite" and rifles through her basket, he takes out the picture of Dorothy and Aunt Em. There is no shot of him "holding" the picture, it's just resting on a dark background.
At the end of "Follow The Yellow Brick Road," Dorothy turns to wave back at the Munchkins. In the lower right, you can see the matte painting of flowers covering the top of the heads of several Munchkins waving back.