When I first took off in beta 16 (patched from 15), the horizon fog effects was messed up and there was just a solid band of color from a little below the horizon to above the horizon. There was no blending at all. It was like this for several minutes as I flew around at low altitude. Then later as I climbed, suddently the horizon snapped back to the correct view with the fog effects and horizon blending. I don't remember exactly what altitude I was at.
Geforce4 4200 128 meg, nvidia 40.72 drivers