Revealing mistakes: When Spritle and Chim Chim are "outside" tanning themselves, the stage ceiling can be seen in Spritle's reflector.
Errors made by characters (possibly deliberate errors by the filmmakers): When first visiting the Racer family, Royalton says one line in German - "Pfannkuchen sind Liebchen" - and translates it to "pancakes are love." "Liebchen" is a very old fashioned German word for lover, the correct German sentence would have been "Pfannkuchen sind Liebe."
Continuity: When Royalton visits for the first time, the third button on Mom's shirt is undone. The next time she is visible, the second button is unbuttoned. From then on, all the buttons are buttoned.