I don't know, as far as I know it's not really happened. But if it follows the same rules as say time travel then it suggests that doing such a thing could disrupt the world as a whole.

[He frowns softly. ]

Pony meddling in that world has already made Equestrian magic leak into it, causing them ever increasing problems. Magic shouldn't invade a world not designed to sustain and survive it. I feel we've given them enough problems as it is, without potentially adding anything else into the mix.