Oh, and going into the weight table, selecting the imaginary number then Shift-right clicking to clear it works. All the badness goes away for that vert. Hmmm...maybe I can script a scan, set the -1.#IO to 0, then clear Zero Weights.
You should definitely report it, even if you create a scripted work around. I've never seen this myself, but chances are other people have and it would be better to have Autodesk fix the problem than have everyone come up with their own custom solutions, especially considering most Max users don't know MaxScript or have…