Character mistake: When the munchkin soldiers first arrive, you can see that some of the munchkins in the front are not beating their chest at the same time. This also happens when the munchkins sing the "fa la la la la" song before the witch arrives. (00:24:25)
Suggested correction: That happens in real life too with real soldiers and live performing actors as much as they try to perfect it because they're human, not machines. Unless there's a rule in the film that a munchkin has to be perfect and can't make mistakes like that, not seeing why this is a movie mistake when this is something that happens in real life.
Continuity mistake: As Dorothy and the Scarecrow dance off and sing, "We're off to see the Wizard" the flowers that were on the fencepost have been removed. The stepladder by the fence has also been moved without being touched. (00:38:55)
Suggested correction: The flowers are still there.
Other mistake: The winds during the tornado were so strong that it ripped a tree clean from the ground but 2 seconds later proceeds to barely blow over whatever she set down. How is she not getting blown away?
Suggested correction: Tornadoes are strange like that. They'll blow one house to destruction completely and then not damage the one right next to it at all. It has to do with how the clouds are formed, the direction the wind blows, and the temperature of where it's sucking up air. It's actually a pretty fascinating study if you'd like to check out how strange tornadoes behave in the sense of what they blow away and what they don't. They're not quite as black and white as they seemingly should be.