i could see this happening in some kind of awkward comedy show like Mr Bean.

Although it looks like a cool idea it's fairly obvious that you used the bag to paint because the underside will leak paint and be discolored. In my experience it's better to just put the cardboard stencil in a thick plastic shopping bag after painting if you want to reuse it.

I don't understand tagging, but I recall seeing this a while ago. Here's more info.