While testing my new map, I had to remove my cfg, to see how the map look slike with default graphical settings.
After I've started Nexuiz with it's own regenerated config.cfg, I've experienced this:
http://www.youtube.com/watch?v=hN-7l-KCbO0
For me, it looks like some kinda cubic clipping. The clipped stuff is a patch cone, projected to the sky with a _skybox entity.
System spces:
Windowx XP SP2
Gef3 Titanium 200 driver: 6.14.10.9371 (latest to it iirc.)
I'll try to launch the game again, just to see it's only for the first time.
EDIT: Further inspection tells me that hihg and ultimate cfgs render the sky correctly.