I decided in the past that interpolation was the big evil-doer. Turn it up high or turn it off (and get some sleep for free, because it takes a lot longer to render). And planes and infinite planes are the first to show this kind of artefacts. I suppose the interpolation works the same as, for instance, the jpg compression method, where calculations are made for a larger part of the surface. The lower the interpolation number, the larger the chunks that are treated as one 'lighted area'.