- don't allow any ceiling portal that has a lower position than a floor portal in the same sector. The will inevitably lead to problematic situations.

src/portal.cpp

@@ -1086,6 +1086,26 @@ void P_CreateLinkedPortals()
 				sectors[i].lines[j]->flags |= ML_PORTALCONNECT;
 			}
 		}
+		if (sectors[i].PortalIsLinked(sector_t::ceiling) && sectors[i].PortalIsLinked(sector_t::floor))
+		{
+			fixed_t cz = sectors[i].SkyBoxes[sector_t::ceiling]->threshold;
+			fixed_t fz = sectors[i].SkyBoxes[sector_t::floor]->threshold;
+			if (cz < fz)
+			{
+				// This is a fatal condition. We have to remove one of the two portals. Choose the one that doesn't match the current plane
+				Printf("Error in sector %d: Ceiling portal at z=%d is below floor portal at z=%d\n", i, cz, fz);
+				fixed_t cp = sectors[i].ceilingplane.Zat0();
+				fixed_t fp = sectors[i].ceilingplane.Zat0();
+				if (cp < fp || fz == fp)
+				{
+					sectors[i].SkyBoxes[sector_t::ceiling] = NULL;
+				}
+				else
+				{
+					sectors[i].SkyBoxes[sector_t::floor] = NULL;
+				}
+			}
+		}
 	}
 	//BuildBlockmap();
 }