Everything Cinderella got by magic would last until 12 midnight. After she noticed that it was almost 12, she dropped the dance with prince and ran away (and lost one of her shoes at the palace).

At 12 O'clock midnight, horses turned to rats again. Her dress etc. vanished too. But why did the shoe at palace remain intact?

In subsequent days, the shoe showed the magic too. It didn't fit on any other girl. So, it is clear that it was still a magical shoe (it didn't turn back to an ordinary shoe, if the shoe wasn't designed from nothing).