Why does Cinderella's shoe always fall off, if it was created to fit to her foot?

This is an interesting question and could have several
different answers:

1. The Fairy Godmother who
   created the shoe incorrectly sized it.  It's possible.  She was old.
   


2. The glass slipper, though referred to as "glass," may
   have actually been another see-through material that expanded (or stretched) over the
   course of the night when the heat from Cinderella's foot was applied to it.  This makes
   sense because a true "glass slipper" more than likely would have cracked at some point
   in the evening, mauling her foot.


3. As she danced,
   Cinderella's foot got sweaty, providing lubrication, and while running her fit simply
   came out.  She was likely barefoot in the slipper, so this makes
   sense.


4. The Fairy Godmother designed the shoe to come off
   at the critical moment.  In her wisdom it was planned all along so that the prince would
   have something to track her by.


5. Cinderella may have
   wanted to lose the shoe herself as a way for the prince to find her.
   

These are my guesses.