I was able to replicate this issue... back light source (...a desk lamp) at the right upper corner of the frame, hand between lens and light source... single shot, AF-S, AF spot... ev'ry try to focus the hand in the center of the frame wasn't possibile, it's always out of focus... the distance of the light source to the hand was approx the same one that was between hand and lens... same scenario (...distances, settings, lens) but w/ the light source out of the frame, the hand, even if slowly, it was focused... this could be annoying, knowing that in some situations the focus it'd not be possible.. it has to know now which are the situations when this could happen, and go to manual focus as I did in this occasion.