Build Mode Feedback for EA

I have experienced a good deal of funkiness related to walls/rooms and foundation myself, Hms. I actually rebuilt my Acrophobia from scratch (once I had the structure completely figured out) because of it, and I'm experiencing some weirdness with my current build too. I have gotten a lot of "object must be placed on floors/terrain" when there is definitely flooring and terrain there. Creating a room around the problem area usually seems to resolve it. However, I have yet to see the "wall doesn't exist" glitch that you mention. Can you tell us more about the circumstances under which it is occurring?