The most dreaded problem with editing - Leaks. In order to vis your level it must be a totally enclosed space having no opening to the 'outside'. If QBSP finds such a leak it will report the error and generate a point file (mapname.PTS). Vis then will not be able to fulfill its job until you find and repair the leak.

The best suggestion is going back to before the leak was created and simply do not create the leak in the first place. You should save your maps in increments, ie; mymap1, mymap2 etc. If you haven't done this and found out that you have a leak you will have to find and plug the leak

Sometimes killing leaks can be a simple process - other times it can be very time consuming. In order to find a leak you need to use the .PTS file (pointfile). Copy over the .PTS into your maps directory. Load Quake and run your map. In the console type pointfile. Quake will create a path of glowing white lines which lead from "inside" your map to "outside" of your map via the leak. Sometimes this can be difficult to find and you will probably need to use the noclip command to fly around looking for them - they'll stand out against the black "outside" of your map. Follow this line until it enters your map. Remember the location of this hole and go back into Worldcraft. Once inside Worldcraft look for a place where two brushes join at the leak. You'll then want to build a large square brush that will encompass the entire leaked area, or you might move the two brushes together so they overlap. You may have to repeat this process until you've filled up all the leaks.

Something else to consider - when Quake is drawing the particles to create the lines to your leak it only allocates enough memory to display 2000 particles. Often you'll have a lot of leaks and you'll get an error about not having enough particles available. You can solve this by running quake with the "-particles XXXXX" command line. Quake -particles 50000 will give you 50,000 particles to work with. The down side is it will require more memory...

(Thanks to Scott St. Jean for this tip.)