Due to some recent dicussion in the TF2maps Steam group chat, I decided to do some testing on what would happen when you texture one of the faces of a world brush to skip and compare it to nodraw. The way I did the test was create a simple room with two same sized brushes both off the ground by a certain amount however one of them has their bottom face textured to nodraw and the other one is textured to skip. At first hand, I thought the brush on the right textured with skip would be considered an invalid solid as skip is ignored by the compiler. So I compiled it. The results were quite unusual. The nodraw brush on the left works just as it should be, however the skip brush on the right looks odd. The faces of the skip brush have streched to the ground. From another angle. But when I walk towards to it, I can get inside of the streched faces. However it's collision is identical to the one of the nodrawed one. What is also unusual is the engine cuts the square under the skip brush as you would expect if you would have moved the brush in Hammer to touch the floor. Now the possible reason behind it is that Hammer cannot detect that it is a invalid solid as it looks like a normal convex world brush to Hammer. However the compiler does detect that it is invalid and thus extends the brush until it meets the nearest brush facing the skip texture. But what it doesn't do is also extend the collision of the brush and that makes it sort of interesting. Perhaps you could use this method to make a secret room of some sort?