A couple tips for fixing areaportal leaks. I learned this while going through my first map using them. If you do not know what areaportals are I would suggest reading this page. They do a tremendous job at speeding up the play on your maps and build times.
http://developer.valvesoftware.com/wiki/Areaportal
1: Hard to find leaks where the *.lin file just shows the line going in a circle or does not really show the leak.
Try removing all props, displacements and func_detail brushes along with any other standard func_brush by deselecting the visgroup to take them out of your view. This will remove the clutter from the map and only show the brushes that can seal the map / portal area allowing you to trace the leak down to the exact location.
2: Make sure there is no func_details brushes breaching the portal areas. If you have any sort of trim or other detail the spans between 2 portal areas it will cause a leak. You will need to split the detail brush in two leaving it to meet on either side of the area portal.
3: When testing maps how to speed up the process.
Trial and error of looking for leaks can be frustrating and time consuming. One tip I found is that when you go to run the map deselect the options for VVIS and VRAD by setting them to 'NO' this will only complete the make BSP portions of the map and show you any leaks. Also leave your game running at just ALT+TAB out of it between map tests. Make sure you 'Disconnect' from the map between tests to make sure nothing caches.
4: Really hard to find leaks. Have an entity that shows to be leaking but your map is sealed.
A: Do a 'select all' and look for origin points that are outside your map and in the void. If you are not using any sort of func_door / func_rotating then simply do a 'Center Origins' option in hammer and it will set them all correct.
B: Goto the Map > Entity Report > Select a group of entities and look for origin points in the void area outside of your map. IE Select all 'prop_static' and then look at the 2D overhead view for anything outside the map.
-Teddy
Fixing areaportal leaks
