Eric Tams: "...trying to create that delicate balance leads to a lot of shuffling of levels and a lot of revisions and tweaks to existing levels. When we started the project making any big structural change in a level or the order of levels would lead to hours or even days of busy work trying to reconnect things and make sure they lined up again. If we ever wanted to ship something the size of Portal with the finely tuned balance we desired then we needed a way to be able to make big changes to the layout of the game without paying the cost of making everything line up again. We needed a way to bend space. We needed to think with portals. Using portals to connect different areas in the world we could make any type of impossible space work out. You could look through a hallway into the next room but the hallway might be on the other side of the map and the room you are looking into might be in a completely different orientation. We could seamlessly insert an elevator, a huge expansive vista, a room that was bigger on the inside than the outside, or even create an infinite fall by connecting a shaft back into itself. Soon every connection between any space was a portal. We would even switch them on the fly. Even a simple door worked like the cartoons - just a facade painted on a wall that seamlessly opened somewhere else entirely. Once the game settled down we were able to finalize our path and remove all of the world portals. There's only one impossible space left in the whole game - see if you can figure out where it is."
Found it. The room in 'The Part Where He Kills You' that Wheatley drags infront of you. Inside, he uses the Crap Turrets on you by mistake. THAT room is impossible. By noclipping inside the room while it moves, you'll see that its just a solid prop. If you portal inside the room, then noclip through a wall, you'll see that its a completely separate part of the map. The chamber exit door then teleports you back into the previous area but at the other side of the room.
EDIT: After looking at the Portal 2 BSP Entity List, i'm assuming that this is a 'linked_portal_door' entity. Probably brush based, at a guess. http://developer.valvesoftware.com/w...ed_portal_door
EDIT 2: Whichever mod changed the title, change it back. The word 'room' isn't a spoiler.