In Aperture map there's a room behind some of the glass walls which, at the end-door there's text on the wall which reads "Do not open!" over and over again.
All of the doors that lead into that room are yellow, our team couldn't figure out how to open yellow doors.
Upon spectating you can go through the doors, however you can't go through the door which reads "Do not open!", however you also can't spectate through the Tubes in the Red-Button Doors. The game teleports you, rather than uses door mechanics; So perhaps going through (Assuming it's possible) the Do Not Open door teleports you somewhere?
Also, near the start of the map - Just past that ... wall of cubes, or whatever there's a trader door; That trader is the only one that has the Companion Cube inside of it.