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."