If you try to export a Max scene as a Source Level with the VMF exporter that has any bitmaps that Max doesn't recognize applied to world geometry or displacements, the export will fail with an uncaught exception. This problem is most notable if you use the Material Library Generator to create a collection of material libraries from VMTs and VTFs with 3ds Max 2012 or older but then try to export that scene with 3ds Max 2013+ (since, as of this moment, there is no VTF plugin for Max 2013).

I am adding some scripts to an upcoming release of WW to help find any offending objects.

The best measure to avoid this problem is to create your material libraries from TGA files instead of VTF files. You can convert VTFs to TGAs with VTFEdit.