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.
Another known cause of this error is extremely complex or "squished" displacements. This is an error that likes to sneak past the compile log. Try looking for a displacement that has points crossing/intersecting each other.
This can also be caused by an overlay on a very large brush. Splitting the brush into multiple, smaller brushes can solve the issue.