ZDoom

This is merely kept open for the bad lamp clipping in softpoly.

If there are overlapping linedefs this will inevitably result in undefined behavior.
OpenGL's depth buffer will simply hide the glitch. I guess the only way to deal with this is to clear the bogus linedef through a compatibility setting.

[spoiler][imgur]https://i.imgur.com/7qIgp3y[/imgur][/spoiler]

I guess, as GZDoom is more and more strict about errors, that would explain it.
OpenGL is far more forgiving.

[b]Tested with:[/b] 3.7.2 (64-bit)
[b]Test material:[/b] [url=https://www.doomworld.com/idgames/levels/doom2/megawads/requiem]Requiem[/url]


In the Software renderer, the wall is "bleeding" through (or whatever) the floor.
[imgur]https://i.imgur.com/crOq2eT[/imgur]

In the Softpoly renderer, the wall is fine, but the lamp seems to be oddly clipped.
[imgur]https://i.imgur.com/kwqKKb1[/imgur]

All is good in the OpenGL renderer.
[imgur]https://i.imgur.com/6QjXMdb[/imgur]