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 Mr. Stork is on the cloud and he looks down on Florida, you can see the cloud through his beak.
Revealing mistake: When the stork is walking on top of the train looking for Mrs. Jumbo, there are clouds moving in the background. After the bear or whatever roars at the stork, a cloud moves to the right of the picture and finally goes off screen- except for the very tip of it. The tip of the cloud stays on screen, even though the clouds before it were moving at an even pace.