When you think about mirrors in fairy tales, especially in Snow White, they get a negative vibe-like in Gilbert and Gubar's essay where the mirror is seen as representing the Queen's narcissism . But I stumbled across a Cinderella variant with a pretty cool use of mirrors, that represent truth and inner beauty more than obsession with self, or could be seen as a way of foreshadowing. In this Hanoverian version of Cinderella, while Cinderella is still a servant, she peeps into a room with a mirror with a golden frame, and sees "a lovely girl radiant in royal robes" and wearing a gold crown, but doesn't recognize the person in the mirror. At the end of the tale, after she's wedded the prince, she looks in the mirror and sees herself, and realizes it's the same image she saw in the mirror before, but never knew until then that it was herself.
I hadn't heard this version of Cinderella before, but this image on deviantart captures the spirit of it.
Tale found in Harold Bayley's "Lost Language of Symbolism" from 1912.