So, How do I fix leaks?
Think of your mapping space as . . . outer space. That is, your map needs to be completely "airtight" if you want it to compile right. Many games use a subtractive method for mapping (that is, cutting rooms, hallways, etc from a giant cube). Half-Life and its ilk is all additive - which means that you need to enclose the playing area completely from the "Void" (or, the places where you have not built).
You can't compile a map with light (rad) if there is a hole anywhere (the engine won't understand what to light and what not to light, so it will render the map completely in fullbright) Also, you can't have an entity outside the map, for much the same reason.
Basically, a leak can be many things. It can be a giant hole in a wall, it can be one of those aforementioned entities. I think what you're describing as the visual error is what's called the "hall of mirrors" (meaning only that, when looking at the 'Void' the engine doesn't understand what to draw, so each pixel just repeats the last thing it drew ad infinitum.) You're probably staring straight at your leak - the Hall of Mirrors is a good indicator of a leak.
First, an example of a proper corner:
And an example of a giant leak (the "void" is the gray):
All that said, could you post some screenshots?