This is probably an easy fix that I'm sure has been asked and solved before, but I couldn't think how to describe the problem. Basically in 2 locations of my map, I can see a room that I'm not supposed to see. These 2 areas are the same, just mirrored. But I only see this error in these 2 areas so I have to assume it's something to do with the geometry there. Here is what I'm supposed to see: But if I move slightly to the side, I can see inside the room: Now here is that same corner in Hammer: I was stood at the bottom of the func_detail brush seen in the bottom-right, looking at the top-left. There were no compile errors or leaks. The rest of the map/skybox is fine, its only these 2 spots. Any ideas?