I apologize if I use incorrect terminology, as I haven’t done any work with Hammer.
Before you ride the drainage pipe down the hydroelectric dam in Surface Tension, I managed to find a few spots where there are no brush faces, and you can see into the skybox.
Starting from here, the left side is where the player enters the Hydroelectric Compound.
If you dive underwater…
You can’t walk through the one on the right…
But there’s a pocket in the one on the left where you’re no longer underwater.
Some other, mostly minor problems. (And by minor, I mean REALLY small)
If you stand on this rock before you reach the dam,
You can see a gap of sky where there shouldn’t be.
And should you stand on the barricade to keep you in the reservoir, and crouch.
Bottomless river.
But there really isn’t a reason for anyone to end up back there, and I only found it since I was missing-faces hunting.