Don't look now, but your skybox is showing!
During testing of something or another, i happened upon some unusual terrain combo that raised you so high above sea level, you could see see the bottom edge of the sky! (box). Unlike some skyboxes, the night time skybox in caveman only extends slightly below the horizon. so its more like the top half of a full skybox cube that surrounds the camera. Works fine, just need to make sure the box is low enough that its "skirt tails" don't show. Needless to say, the skybox has been moved down to correct this graphic artifact.