That's similar to the one about the wicked queen in Snow White, the bit where she thinks she's seen Snow White off with the poisoned apple, so she asks her magic mirror:

Mirror mirror on the wall
Who is the fairest of them all?

At last the magic mirror tells her what she wants to hear, but, unbeknown to her, a servant is listening through the keyhole. When the wicked queen has left the room, he sneaks up to the mirror and says:

Mirror mirror on the door
Make my penis touch the floor!

And his legs dropped off.