Was Prey designed from the ground up to use portal effects? Because Valve, of course, hacked it into their current engine. In the commentary they even admit they couldn't get it efficient and effective so they gave up and used object cloning to have two instances of something that is touching the portals' fields. Hax.
Fun: Put a portal on a wall with the other portal on the floor right beneath it, and drop in a (companion) cube. More amusing to watch than earning the 30,000 feet award.