Revealing mistake: During some scenes, straw-stuffed Scarecrow's pant legs and his boots would sometimes have a gap that reveals the skin of Ray Bolger's leg. Two examples are, at Emerald City when Scarecrow places Lion's "cape" on him, and also when Dorothy and her friends are told the Wizard says to go away. (01:05:50 - 01:07:25)
Revealing mistake: When they are feeding Annie the Elephant medicine with a wooden spoon, there is obviously no fluid in the spoon.