Photokina 2012: Nikon talks about the D800's outer AF issue

I think many of us were suspicious of this since calibrating AF is normally very straight forward. You measure each AF deviation, then offset each register in the cameras ROM. Since technicians were unable to do this successfully, it meant the deviation was greater than the max value allowable.

This also explains how some cameras were returned with the left AF corrected, but with all the other AF points off . Once you hit the offset limit with the individual AF, you must use the Global offset to adjust further, which shifts all of the AF points.