Dafuq? The screenshot says it all really.

Appears to be a ZDoom bug. Only happens in software, as far as I can tell -- it disappears and reappears as you walk around, too, which is super-nutty.

I've ran into this before... this happens when you wrap a midtexture that has a visible upper texture as well.

Push the grating a bit into the alcove, it'll fix it.

That'll work in the interim, but this highlights an engine bug that needs fixing at some point. I'll whip up a test wad and report it soonish.

I went ahead and addressed it anyway in the latest build. Hope I remember to report this too.