Watch carefully in the close-up scenes of the Emperor in Return of the Jedi. If you look at the left side of his hood (his right, your left), you can see a little patch of black that doesn't match up with the rest of the black. That was colored in to hide the effects used to make Palpatine's eyes yellow. I think it was George Lucas who originally passed it off a Sith slug that feeds the Emperor's power to him or something like that.