Including the ground in places where it's covered by trees from a normal angle, which means it'll show whatever the renderer throws at you when there's nothing to show. Probably whatever was at that location at the last screen refresh, which means the ground and/or special effects will sort of smear over all the holes every time the camera moves or a spell passes over the area. I know that's how TF2 does it (I was experimenting making maps and forgot a skybox - it looks really weird).