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.
Visible crew/equipment: Spade returns to Brigid O'Shaughnessy's apartment and tells her about his meeting with Joel Cairo. Right when he says, "He offered me $5,000 for the black bird," the shadow of the boom mike can be seen above the fireplace. (00:31:55)
Continuity mistake: At the beginning, Belle dismounts from the horse and gets in front of Mammy Lou. Her hands are lowered in the wide angle but raised in the closer view. (00:04:00)