I started on a map today, and this pop out of nowhere, what the hell. This is the first time I ever messed with round ceilings, Hell, first time I messed with anything round at 32.
It could be a result of an invalid solid, sounds most likely since you can clip through it and the arches look suspect, but hammer occasionally does this cute thing where it creates a brush invisible in hammer, but there when you compile, or at least it used to. I haven’t seen it happen in quite some time, but anyway that’s beside the point.
I have had them created from square brushes before. I nicknamed them ghost brushes. Find the brush that is causing it, and remake it, that should clear up the problem.