In the wpisode where D.J. gets a Super Nintendo from his grandmother (it's also the one where Roseanne hurts her back and has to stay in bed), Dan is playing a Nintendo game. When he asks Mark what he has to do in the game, Mark speaks of skulls and blood. Everyone who has played SNES before knows that the music coming from the machine is from Super Mario, which has no skulls or blood in the game.