In the Gm construct map, go to the garage place and look under the path at the side where you can see the hl2 symbol. Fly through it and you come to a small room where there is a couch, a sink a doll on a table and a notice board.
Make a combine so small it can fit in a coulored box. Kill the Combine, and then it will enlarge to the normal size as a corpse. The corpse will get a body part or limb stuck in the box. Then it will glitch out, from kicking you in the groins to sinking through the floor.