I think that "Mirror Mirror" has been paraphrased by countless shows, and so therefore people now remember it as that. When in fact it clearly is magic mirror. Like you say, an episode of another show, used "mirror mirror", which just proves that they got it wrong as well.
Like you say, it is an interesting subject; and one that fascinates me :-)
Cg
Oops, bad comment.
Ok wait what about the line in Forest Gump... I remember it as "Life is like a box of chocolates" but now for some reason it is "Life was like a box of chocolates"