Revealing mistake: The stepmother's arm disappears for a split second when she tells her daughters that there is still a chance for them to become the princess.
Revealing mistake: When Cinderella is about to be locked in her room by the stepmother Cinderella sees her in the mirror. The image in the mirror is a reverse image not a mirror image. The door is seen opening on the wrong side.
Answer: Add in that how many possible glass slippers could there possibly be? It would be an incredibly uncommon type of shoe that would be amazingly difficult to make.
LorgSkyegon