In one of the "Tree House of Horror" Halloween specials, one of the short stories is that Principal Skinner and the faculty are cooking the kids and eating them. At the end after Lisa, Bart, and Milhouse escape from Mrs. Krabapple's room, they are being chased by Skinner and 3 others. The slip up is when Skinner is going after them you can clearly see he has his apron on ready to eat them, but in the scene right after that, he doesn't have it on. He then puts it on saying something along the lines of "I think I'll start by...eating your shorts." So again, the slip up is he has his apron on, then doesn't, then puts it on.