Pac-Man
Enter escape-tunnel, quickly go up & down in tunnel a few times
without coming out. Go down as your last move and Pac-Man will "wipe"
into the screen. He can now move up & down in this vertical plane
through the walls, but not sideways. Hey may eat vitamins and get
killed by ghosts. (Not particularly useful, but interesting.)
Frying can cause the walls to starting "glowing" at irregular
intervals during the game. Game play is otherwise the same, but its
pretty annoying (consider it another level of difficulty