Hell revealed 2 skybox seeping through floor

[Apeirogon]

Download hell revealed 2
https://www.doomworld.com/idgames/themes/hr/hr2final
Start gzdoom with no monsters flag. Type in console map map08 and then warp -480 250 64. Turn to the left until you looked directly on red door/door which require red key. Move three steps to the right. You should see little gap on the floor with skybox in it to the left from you.
https://imgur.com/a/FcHyAS0

[3saster]

This is not a bug, but the result of a certain trick called Shadowcasting, which allows floors and ceilings to have different brightness levels in vanilla. Unfortunately, similarly to the deep water trick, this does not always display correctly in OpenGL, hence causing the bug you see (this is also present in the exit area of MAP01 and the western section of MAP27 in front of the Revenant/Archvile cages). Boom added a flag to allow different brightness levels in a non-hacky way; I've been meaning to use that and the automatic level compatibility to correct this bug, but haven't gotten around to doing it, it's not a trivial fix. I think I'll post some pictures in the "maps that need compatibility settings" thread, maybe someone else might be interested too.

[Apeirogon]

Ah, I suspect some eldritch renderer terror somewhere since map editor shows that there are too low/high sector floor/ceiling near near that buggy point, but didnt know at what I should look to see it.

[Graf Zahl]

Stuff like this happens when mappers use software rendering hacks without fully understanding their nature and how they affect node building.
Sorry, but in this case the only fix would be a total reconstruction of the hack - it would have been easier if the creator of this map hadn't used the same hack sector for all windows, but for this to get fixed, each slice of the hack would require its own sector. Way too much work.