Nikon has announced a service advisory for the Nikon D810, in response to reports of 'bright spots' at long shutter speeds and in 1.2X crop mode. According to an announcement on Nikon's website, affected cameras will be serviced free of charge.
Apparently, the issue only affects cameras within a certain serial number range, and if you're lucky enough to have got hold of a D810 already, you can check the serial number of your camera against Nikon's database via the company's service and support pages.
Nikon USA Service Advisory
Nikon EU Service Advisory