I saw this too when i copied a dungeon and then started making changes. It has to do with portals. The game thinks that wall is in another 'portal' or 'area' that you can't see and so it is not rendering the wall as an optimization.
Read up on this:
http://www.creationkit.com/Room_Bounds_and_Portal_Basics
and this:
http://www.creationkit.com/Bethesda_Tutorial_Optimization