Vis Decompression Overrun
This compiling error is a tell-tale sign that there are far too many visleaves in the target map. A visleaf, or simply leaf, is generally a world brush. This means that there is very detailed world brushwork, an enormously large map, or a mixture of the two. To attempt to remedy this problem, review the map to see if it is possible to break up the level or cut back on some leaf-usage through the use of detail geometry.
This error can also occur through the placement of a large amount of world geometry in the 3D Skybox.