Any version of ET has this vulnerability AFAIK.
On certain models and structural brushes, players can clip curves and get inside of the model. In this case, players jumped onto the ledge and fell prone to clip the curve. On Raiders, if you hurl yourself at certain parts of the oil tanks, you can ‘fall’ inside of the model and shoot players who are outside, but they can’t shoot you back, except for with the flamethrower in some cases.
A good demo showing a player trying to overcome my bugfix is found here:
http://clanoxygen.com/hiroshima/railgun-fixed.zip
You can see how he can no longer get in far enough to hide inside, but at points you can see how it looks from the inside, as you can see out and shoot out from the inside, but you can’t shoot or see into the tank.
I don’t know enough about modeling to explain why some objects are ‘penetrable’ and others are not, but having looked at a lot of the stock maps in Radiant, I can tell you that I see plenty of clip brushes accompanying many of the models. Adding clip brushes doesn’t hurt, but I imagine that they would add up if used excessively.
I have noticed that when structural brushes get too thin in Radiant, like some of the walls in Supplydepot for example, they become vulnerable to some effects from either side of the wall. For example, the crane controls in supplydepot could be satcheled from the other side of the wall. A thin blocking brush had no effect, so in the end I had to make one that was 6 units thick IIRC in order to completely block the effect of the satchel.
That fix, BTW, is found here:
http://bani.anime.net/banimod/forums/viewtopic.php?t=5932
–Mortis