There's a graphical glitch in places in Hexen where several consequent sectors are "waggling". Since their linedefs do not have "lower" textures, engine renders these "gaps" with sky or anything that is positioned behind the area.
You may see this clearly on Hexen map08 (swamp) in the hall with dozen of waggling floors (in NW part of the map), and on map 40 (dark crucible) where the bridge is waggling.
I suppose this also persisted in original Hexen, but its renderer used to copy neraset floor texture on these gaps so this glitch was unnoticable.