As seen in this image (click it for a larger view), when viewed from a distance the sky comes through where slopes are to be. Moving closer, the slopes are rendered properly. I'm just wondering if that is a bug or not.
The slopes are so steep that their z-position overflows at the player's viewpoint. This means that half the math in the code needs to be changed from fixed point to floating point. I'll have to see what's possible.
Fixed. However, I can't guarantee that there aren't other places in the engine where such an overflow may occur so even though it renders properly now there's still a chance for this producing glitches.