I checked my D600 after about 300 picts and found a few dots on the upper left of a test image, not sufficient to cause alarm. Today, I ran off 4,000 more shots with the camera in various positions (using the interval timer). A new test image showed lots of dots, concentrated on the upper left, as well as what looks like a dusky smear over approximately the left third of the viewing field. Looks pretty alarming now. I'm going to send mine back to Nikon tomorrow. (Serial number about 3,000,000.) Seems a shame to have to pay postage and insurance.