Within the map, vertex coordinates are only stored as integers. In normal map editing, vertices may only be placed at integer coordinates. During BSP building, however, lines may need to be split into segs at a location that has non-integral coordinates. The BSP builder ends up having to chop a significant amount of precision out of the coordinates of its generated vertices, and as a result, segs may end or begin far short of where they need to be placed for a mathematically correct BSP. This results in an open subsector, and the flat of the subsector will bleed out at the location of the offending vertex. Any open subsector will display similar behavior.