I discovered this bug on Accident Prone by Shmitz, but it is a bug that exists in all maps, including the levels in the original game (try testchamber 17).
Here's an example photo (taken from Accident Prone, because it has a suitable area, again this bug is NOT particular to this map)
http://img159.imageshack.us/img159/3721/accidentprone0002rn8.jpg
Both portals are against the top edge of the walls.
-If you go through blue portal and then walk off the edge above the blue portal, you are teleported to above the orange portal.
-If you go through blue portal, and replace the orange portal to a floor, when you attempt to walk off the edge above the blue portal, you get stuck and can free yourself by crouching.
-If you go through the blue portal, and replace the orange portal to a ceiling, when you attempt to walk off the edge above the blue portal, you get teleported into the ceiling above the orange portal, and you are stuck permanently (until you noclip out).
If you use portal bumpers this bug is fixed
I tested this in a map I made. If you make a brush (texture "invisible") tied to the entity func_portal_bumper, the above actions do not happen. Orient your portal bumper in the following way:
(hint: you can make the portal bumper brush smaller so long as one of its corner edges is touching the corner edge that you wish to bump portals away from)
Maybe once Valve knows about this (if they don't already) they'll fix it.
Just something to look out for if you want to keep your maps bug-free.
