Revealing mistake: When the elephants are making fun of Dumbo in the train caboose, Dumbo's mother pulls a pin which causes the stall door above the other elephants to fall. When the door bounces back up, you can see that the animators forgot to draw the elephants' heads and trunks and only their bodies are visible.
Revealing mistake: When the notice of impending strike is posted in the town, the American spelling of "labor" is used. A Welsh signwriter would have spelt it "labour."
Revealing mistake: To save money during the beach picnic scene, Welles used stock footage from King Kong (1933) for the jungle background. You can quite clearly see pterodactyls flying in the background (confirmed on DVD commentary).
Revealing mistake: When Larry is attacked by the werewolf, Bela, there are two shots when they are behind a tree, when he is actually fighting a man. (00:15:40)
Revealing mistake: When Alvin and his mule get struck by lightning, if you look closely you can see the hair flip up on the mule's flank as he's hit with a whip to make him get up, and you can see the whip drag off to the side of the screen.