Mike Judge was slated to record a cameo as Beavis and Butt-Head (1993) for the episode "Apocalypse Not", but a lawsuit erroneously accusing B&B of encouraging children to start fires prompted him to pull out at the last minute (that particular episode features scenes of mass destruction when Duckman mistakenly believes he's survived the end of the world and goes about trashing downtown).
Stuart Pankin of Dinosaurs (1991) fame plays the unnamed character who says "This isn't a biosphere, it's a sewer!"/"I don't know what feces are, but they smell like crap!"