Why is my ceiling semi-see-through?

P.S. It only seems to do it sometimes when I test the map, but not others. I have yet to find a cause.

1 hour ago, _koraichu said:

P.S. It only seems to do it sometimes when I test the map, but not others. I have yet to find a cause.

Try making a triangular sector in the corner, it could be node building issue b/c of that geometry behind...when I do lots of curved pillars I have to split sectors, draw hidden lines sometimes because of rendering errors. In this image you can see the box sectors around the pillars and the sector inside the 4 diagonal spaced pillars I needed to place b/c there were holes visible from certain angles.

Yeah, if the ceiling isn't F_SKY1 and the way you see the sector is splitting off in triangles that aren't geometry you've actually drawn, it tells you there's a nodebuilder void there. You usually solve it by either moving some vertices around, deleting and re-making sectors, drawing lines through that sector, or building nodes later on. It could also POTENTIALLY be an unconnected vertex. Make sure "Snap to Geometry" is turned on and that vertex is actually touching the wall there.

What is this vertex doing here?

Is it providing any kind of function?

Curious to see if the problem goes away if the vertex goes away; please update this post if so.

In addition to the node theory, what happens if you analyze the level? By default you press F4 in UDB or press the check mark icon.

Modern source ports are pretty good about dealing with minor errors, especially GZDoom which you mentioned. They are likely to show minor visual glitches rather than crash or fail to load a level. But fixing the root cause is always a good idea, whether a level error or a node error.