Publicado originalmente por Zappy:

In the Source export tab, either a bit below or a bit above the export button, should be a "Weight Link Cull Threshold" thingie. (Not being home, I can't check right now.) Just slide this all the way up to 1.0. Note that this only appears if you're exporting as DMX... but then again, this issue is non-existant for SMD models.

Or if you want to fix it manually, just make sure there's not a single vertice of the model that is rigged/weighted to more than 3 bones.

I think The cull link threshold feature option should be avalible on SMD because GoldSrc format will not export if there is any weight variation across bones; I wouldn't say its a "non-existant" issue.