Build a room with 2 hallways next to each other but separated by a thin wall, going opposite direction and connected at the ends. Now texture the walls with a sky brush. Add some lights and play the map. Watch the wall facing the hallway on the other side when you get towards the end of the first hallway. Towards the end you can see through the sky and into the hallway beside you! Dude, its a friggin ingame wallhack effect!!!