i've noticed this happen on some of the walls on offical maps like 2fort and hydro. today i noticed it on one of my own maps. it's something i want to avoid, as it allowed me to get an easy headshot as someone came round the corner. i read something about brushes less than 16 units causing players shadows and models to show through on the other side, but the wall in question is 16 units thick. this is game breaking in my opinion, and something i'd want to avoid. but since i've seen it on valve maps, i'm not sure if it's something they missed or just a nuance of source that has to be lived with and worked around.