Downton Abbey costume designer Anna Mary Scott Robbins was tasked with creating costumes for a Fashion show in Downton Abbey's fifth season. This dress was reused intentionally - first on the model, and then again on Lady Mary after she purchases the piece.
"We bought a beautifully embellished tea dress, that was nigh on a hundred years old, that was very delicate, and we spent a couple of weeks restoring it, re-beading where it had fallen apart and re-backing it to make it more robust. It was black and white, entirely beaded with flowers on it, with sort of scallops on the bottom. And it's quite cutting-edge because it was just below the knee. We put [Lady Mary] in the beaded dress for the cocktail party. Lady Mary.... exclaims 'Yummy!' about the beaded dress."
You can read more about Downton Abbey costumes at the Slideshow: Secrets of the Catwalk article.