Continuity mistake: When they first meet the Tin Man, the Scarecrow is holding an apple in each hand. He drops the one in his right hand but the other one disappears when he reaches for the oil can. Later when they all walk away both apples, and the one that Dorothy dropped before, are gone. (00:44:05)
Continuity mistake: The Munchkin Mayor's watch keeps changing time, from nearly to the hour, to twenty after the hour.
Continuity mistake: When the companions are walking down the corridor to the wizard's room, Toto is on their left side. Then, the camera view suddenly cuts to being behind them, and in which case, Toto is suddenly on their right side.
Continuity mistake: In the beginning of the movie Dorothy comes running into the farm and there's an old tire hanging from a tree. Two shots later as she walks away from Aunt Em and Uncle Henry the tire has been moved. The angle at which it hangs has been changed and the piece of rope hanging on the bottom of it is now three or four inches closer to dead center than it was before. (00:03:00)
Continuity mistake: As Toto escapes from the Wicked Witch there's a mortar and pestle to the left of the hourglass. In a close-up Dorothy looks at the hourglass after the Wicked Witch turns it over and we see the pestle has been moved to the other side of the mortar. There will be no confusion about this if you look at the direction of the shadow cast by the hour glass. (01:18:20)