Nikon has issued a service advisory for the D750 as a result of a shutter malfunction that affects some units manufactured between October and November 2014.
In the affected units, the improperly functioning shutter may result in shading a portion of the images capture with the D750. Nikon is offering to fix this issue for all affected D750 camera free of charge.
Nikon has provided a way to check if your camera is affected by providing your serial number here on Nikon’s website.