I think you can pretty much ignore this warning, you can still compile and run the map just fine with it.
But if you get more than one leaf portals saw into leaf warnings, you want to do something about it. Here's what
Tommy's error page has to say:
Basicly this comes from an area whose floor is not square enough, square floor areas never give this problem. Note: often if there is only one leaf saw leaf in a map level, it can be ignored. So before getting busy, see how your map plays!
Solutions to try when you have a lot of leaf saws, or problems from leaf saws:
1. VIS -full compiles were designed to help with this problem, so the FIRST thing to try is recompile with VIS -full and see if it goes away.
2. func wall any brushes within the coordinates given for the problem, this will simplify VIS in the area because func wall brushes are invisible for VIS.
3. Move walls in the area to make the floors more square.
4. Put a HINT brush across the trouble area. HINT brushes cut VIS leafnodes, so they will reshape the area and may fix the problem.
5. Finally just start ripping brushes out of the area.
Fixing Leaf saw error has also been known to fix other errors in later RAD, like
FindTransferOffsetPatchnum returned -1 looking for patch 1570 in patch 199's transfer lists and other problems.