I've drawn a hemisphere-like shape and it seems to be drawn okay by software and GZDB, but fails to render properly under OpenGL.
GZDB (aka what's expected):

Software (of the same GZDoom version, with vid_renderer 0):

OpenGL:

Minimal example with this shape: http://www.mediafire.com/download/qp4pw ... e/ART2.wad
Happens in both 32bit and 64bit versions.