Been some time since I was really into mapping. I forgot about this easily overlooked issue when using glass textures (may apply too mirror texture too, I forget).
I made opeinings for windows and then used the same size brush to fill the hole and make a window (using glass texture of course). I went looking around in the map and stood in the windows sill. I could see right through the wall and down through the floor and cieling where the window had been placed. If outside you wouldn't be able to see me, but I could see you. Not to mention, it looked like shit.
I remembered that you have to make the window smaller than the opening ALL FOUR SIDES. I make it 2 units less on EACH side.
Little things like this are why I always go in the map and inspect everything I "build" before continuing. Otherwise you may repeat the mistake many times and have to go back and fix more than needed.
Hope this helps someone.