1. Why are Leaks Bad?
2. Good Mapping Habit
3. Reasons for Leaks

What Are Leaks?

Leaks are the natural enemies of mappers. For an additive editor, like Hammer, the compiler must have a finite, all-inclusive space to calculate visibility and lighting. Otherwise, your compiler would spit out a full-bright map, even if you have the most excellent lighting set up inside your map.

When I say an all-inclusive space, I mean the world you're creating must be completely sealed off from the infinite void outside the map. So if you make a map that has a small slit that you didn't notice before compiling:

A gap between solid brushes

Your compile message will warn you of the leak, and your compile time will most likely be a lot shorter, because the compiler skips the VVIS and lighting (or VRAD) calculation completely. And when you load up the map in-game, you'll notice that it's full-bright.

What's Wrong with Full-Bright?

Now, you might say "What's wrong with full-bright?" You might be the kind of cheery person who likes everything lit up 100% everywhere. Unfortunately, not only does most people think it looks ugly (it destroys all the subtle lighting the Source engine is capable of rendering), it also hinders the engine when it renders the map in-game.

If you have a complex enough map, you'll probably notice the map is very "laggy". When you walk around inside the map, everything just drags. With a extremely simple map, you won't notice this. This performance issue is caused by VVIS not running. When VVIS is not calculated, the rendering engine will not have any visibility information available with which to limit itself, so it will attempt to render the entire level. This will most certainly lead to high polygon counts and may lead to sections of the map to render incorrectly or not at all. In addition, full-bright is something not any mapper should condone or allow for their maps. It's simply not an option (kind of like going to school or work naked...it's illegal).