Right. That's what I thought. The problem lies in that the box was designed to close only on a rebuild. Since you didn't change anything, there was nothing to rebuild, so the box stays around for eternity. I think i have a solution for that.
I recently had a look at catia's NURBS implementation, and i must say, that they're remarkably similar. In that you create a nurbs surface from another face and then they aren't connected anymore; you can delete the original face and the nurbs still stays. But I guess you could still call mine the "really crappy version" ;)