Okay, so Princess Celestia asked Twilight if she could create a spell to become human. She does the spell with her friends and the princesses for support, but the spell is more powerfull then tought. And since ponies dont have clothes.. well…… On the other hand, you Anon (pm if you want an other name) doesn't know what's going on and walks in on a rather…. interesting scenario…….

(I might put it in mature later if it gets spicy…..)