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: Cinderella's entire outfit was magically created by the Fairy Godmother. Not only was the spare shoe Cinderella had in her possession identical to the other one, it likely would be charmed so that she is the only one who can wear it. It's similar to young Arthur being the only person who could pull Excalibur from the stone because he was the true king. No other woman who attended the ball wore glass slippers. If they had, they'd be able to also produce the spare shoe as proof.
raywest ★