WHOA...I just found a BIG game-wide rendering problem. I don't know if the devs are aware of it or not, and if they are, whether they care about it...but it can REALLY make your level ugly if you aren't careful.
Look at this screenshot of a level from Tortured Planet, Drowned Downtown Zone Act 1:
Now, notice what happens when a water block is applied to that sector.
It seems that whenever a water block is applied to a sector which borders the thok barrier, it stops the level from rendering anything beyond the inner wall of the thok barrier. This might occur with other FOFs, too, but I haven't checked.
Look at this screenshot of a level from Tortured Planet, Drowned Downtown Zone Act 1:
Now, notice what happens when a water block is applied to that sector.
It seems that whenever a water block is applied to a sector which borders the thok barrier, it stops the level from rendering anything beyond the inner wall of the thok barrier. This might occur with other FOFs, too, but I haven't checked.