One of the joys of rehabbing an old house is uncovering what was done by those who came before. In painting a small bedroom we found three different papers on the wall. Actually all of these were hidden under a fourth wall paper -- a solid rattan kind of paper. That paper had been painted over in green and then tan.
I think there should be a rule that prohibits more than three layers of wallpaper. Kind of like a roof. If you get to three layers of wallpaper, you have to tear it all off and start new.