Nikon announced in February 2016 that the shutter in some D750 bodies manufactured between October 2014 and June 2015 may not function normally, sometimes resulting in shading of a portion of images. In the July 2017 update, Nikon stated it had since learned the same issue may affect bodies manufactured from July 2014 through September 2014 and from July 2015 through September 2016.
Any D750 in those ranges is eligible for inspection and shutter replacement, verified by the seven-digit serial number on Nikon's checker. Nikon examines and replaces the shutter and returns the camera free of charge, even if the warranty has expired.
This is one of the clearest cases in the archive of a fault whose true scope only became visible as reports accumulated: the affected window grew from nine months to a span covering July 2014 through September 2016.