The latest zombie cameo in the show was the premiere of the second half of the fourth season. If you blinked, probably lost, but during the scene where Michonne was walking among a group of zombies, one of these was a little more rotten than the others, with this skeleton almost completely visible. This different walker was a tribute to "An American Werewolf in London", and totally zombified corpse of Jack Goodman. A beautiful tribute to Rick Baker, responsible for the amazing makeup in that movie.