D750 "Banding" problem explained
Even though it's only been on the market for a few months now, some users are reporting an issue with their D750 cameras that causes a dark band across the top of the image when shooting directly into a light source. The shadow seems to be caused by an autofocus sensor that protrudes too far into the mirror box, and appears to be a production flaw that only affects a limited number of cameras. Here's a good explanation of the problem over at Petapixel.com - Nikon D750 banding issue explained. Nikon has yet to make an official statement regarding this issue other than to say it is investigating.