I once had a similar effect but then with some envmapped-shader. As long as it's not appearing after calclighting, it shouldn't be such a big deal. Although you could always inform Eihrul about the happening. He's the one which could actually do something about the case.

Next time you make a video [wich is very a very good thing] take a few minutes to Cut it eh?

optimizeblendmap - command ?
or maybe try doing a calclight or saving and then reloading the map
or an optimizemap

What it looks its doing is applying a blend map to the light map layer creating a dark surface (because the light map is at the ambient color), you posible could resolve this by switing into full bright mode although this would probably not show your lightmap or your blend map. Doing a light calc and then and optimizeblendmap should solve the problem. Ive had similar problems and doing light calc fixes the problems for me at least. Always use a fully lit map when editing blend maps to avoid this problem.

chasester