^ yeah i didn't know that either Jeff
Make sue the origin x of none of your point/brush entities lies outside the void, and also make sure you didn't accidentally tie a brush that seals the world to entity.(entity report, filter by brush-based, select all and hide, and check to see if your world is still sealed)
Another helpful way to isolate leaks--especially on big maps--, is the "big-block" method, using large world blocks to mask off parts of your map, compile, check for leak(s), while reducing the size of the block each time narrow down where they are coming from.
Last but not least, be sure to Alt-P(check for problems) before each compile, because some errors can cause leaks, e.g., a really brush really messed up by VM, or a moving entity without a proper origin will place the origin at the center of the world, and if that area is exposed you may get a leak.
All else fails upload to Problem Maps? life is too short to agonize over leak hunting