I don't know of any offhand that are about leaks in general.
But go here, these are really good starter tutorials: http://richdiesal.jedioutcastmaps.co...als/index.html
As far as what a leak is, well, the virtual "space" you construct your level in, is called the VOID. You can never allow the player entity (or should allow any entity) to be able to enter it or be exposed to it.
It's like, think of your map as a flying spaceship... if there were any cracks or openings out to space, the whole ship would decompress and be destroyed.
Any, like I mentioned, go to File > Pointfile...
and follow the arrow(s), the arrows will goto any leaks in the map... from there you can "seal" them up, and you should be able to compile!