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.

Do any of these settings (extracted from differences in high.cfg compared to normal.cfg) individually fix the sky?
r_bloom 1
r_water 1
r_shadow_realtime_dlight_shadows 1
r_drawparticles_drawdistance 2000
r_drawdecals_drawdistance 500

The only ones that seem likely to have any effect are the first three (but none of them should - but that's the nature of bugs).

I need you to narrow down the specific cause.

Thx, I have found it:
r_shadow_realtime_dlight_shadows 0 makes the hole.

Fixed in engine svn (q3map2 lies about the map size, due to the lightgrid brush), the r_shadow_realtime_dlight_shadows 1 workaround only works because of the infinite farclip matrix used when shadows are enabled.

Thx