Nikon has released a statement to reassure users of the D750 that it is 'working on measures to address' an issue that some models have with unusual flare patterns that can occur when bright light sources are just outside of the frame. The problem seems to occur only in certain units, and only when the light source is at a very narrow range of angles relative to the camera. The flare occurs at the top of the image, with a shadow in the flare manifesting itself as a darker band across the top of the frame.

The team at Imaging Resource investigated the problem and found that it also occurs - albeit to a lesser degree - in other high-end DSLRs like the Canon EOS-1D X. Their tests point to internal reflections within the AF module at the bottom of DSLR bodies as the cause of the problem. Although a light baffle typically covers at least a portion of the AF module at the time the shutter is released, some light can still enter and be reflected by the AF module. Variations in the placement of AF modules appear to be creating mixed results for D750 owners (our own review unit didn't display this problem during our testing). 

See the full statement from Nikon below:

Thank you for choosing Nikon for your photographic needs.

We have received indications from some users that when photographing scenes in which an extremely bright light source, such as the sun or high-intensity lighting, is at a certain position along the top border of the frame, flare with an unnatural shape caused by the reflection of the light may sometimes occur in images captured with the D750 digital SLR camera.

As we at Nikon are constantly striving to ensure the satisfaction of our customers, we have released this announcement to inform users that we are already working on measures to address this issue and will, of course, keep you advised of these as they occur.

Please direct inquiries regarding this matter, contact Nikon Customer Relations by phone at 1-800-Nikon US (1-800-645-6687), 9AM–8PM EST, Monday to Friday (closed certain holidays) or online here.

Once again, thank you for choosing Nikon for your photographic needs.

The D750 statement on Nikon's website.