The bug occurs due to a logic bug in the code for handling skies. In Doom, if a linedef bridges two sectors with different ceiling heights and has no upper texture to be displayed in the space between the sectors, the ceiling texture of the lower sector appears to "bleed" into the empty space. This is used to deliberate effect in outdoor areas where a special sky texture (F_SKY1) is used for the ceiling texture. This allows the sky texture beyond low outer walls to look like real sky, as opposed to a sky wallpaper. From PTR_ShootTraverse in p_map.c: