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)
Continuity mistake: When Casper the Caveman takes off his caveman's outfit to go swimming, he puts his outfit near the edge of the bank. After Daffy says that he can't swim, he reverses back onto the land and goes and collects his suit, but this time the suit is now much further away from the bank.
Continuity mistake: When the dog in the audience has his feet crushed by the hippopotamus sitting down on his chair, you can see that there is an empty chair next to the dog, but in the next shot the chair has disappeared.
Continuity mistake: During the Blue Birds in the Moonlight song you can see Gulliver holding his drink. In the next shot while he's laughing the drink has disappeared.