http://www.nikonusa.com/en/service-and-support/service-advisories/ibs1kfdg/technical-service-advisory-for-users-of-the-nikon-d750.html
In February of 2016, we announced in an update that the shutter in some Nikon D750 digital SLR cameras manufactured between October 2014 through June 2015 may not function normally, sometimes resulting in a shading of a portion of images. Since that time, we have learned that the same issue may affect D750 cameras manufactured from July 2014 through September 2014 and from July 2015 through September 2016.
For those who have already purchased a D750 and would like to have their camera serviced for this issue, free of charge, Nikon service centers will service cameras as indicated below. We sincerely apologize for any inconvenience this issue may have caused. . ........................
