I thought it was one hell of an allergic reaction. Mace was wearing pretty strong aftershave that dark day. ;)

Seriously though, when I first saw Palpy in '83, I thought that was what being on the dark side does to you. It's like a cancer of evil. (His advisers look kind of pale and ghastly too.) The Empire being so human centric, I felt he couldn't be some sort of humanoid alien.

Lucas had to go and muck that up in ROTS, of course.

Palpy claims he's scarred, but is that the official explanation? It seemed more like an illusion melting away he couldn't maintain any longer. He certainly doesn't seem too upset about being stuck with that mug. Does Force lightning really cause instant tooth decay?