This page lists various encountered errors, that may occur during the VIS .bsp-optimization phase.

The following games that QuArK supports, are known to use a VIS .bsp-optimization phase:

Your map is not "sealed" : it means there is a hole from the "inside" of it to the "outside". Look carefully for this hole. Note that QBSP sometimes creates small gaps between polyhedrons even if they are perfectly glued together, if the separation is at a non-90 degree angle. To prevent this, you can make the polyhedrons overlap a bit. Usually, QBSP doesn't mind about overlapping polyhedrons. However, if you get an error "new portal was clipped away" from QBSP, it means you overlapped them too much ! See below. Not easy... If you can't find the hole in QuArK, try running Quake anyway and walking around your map until you find the hole - the "outside" is gray. There is also a console command that can help you : in Quake, you can type "pointfile" and it will leave a trail of dots from the outside to the first entity it ran into.