Short description of error
It seems that using anisotropy produces black odd artifacts under certain mesh circumstances. I don't think this happens often as it seems the actual mesh caused it, possibly by being a little bit too symmetrical?
I suppose often it goes unnoticed so I thought it might be good for someone to investigate upon if there isn't some inconsistency.
Rotating the object by the tiniest bit helps (probably helps by something being non 0 ^__^).
Moving random vertices randomly produces different results that are often completely correct or at least 50% correct. Good to try that in Rendered mode.