After complaints made by a fair number of users of Nikon's latest DSLR, Nikon D810, that they had encountered certain 'bright spots' in long exposures and the 1.2x crop mode, Nikon issued a statement on its official website that the cameras which had experienced this issue would be serviced free of charge.
In addition to a profound apology, it was also stated that this issue arose only with cameras with a certain serial number. In case you own a new Nikon D810, you can enter your D810 serial number on this website and it will be compared with those that have been affected by this issue. Should your camera belong to the group of those problematic, you will be supplied with additional instructions.
If your camera possesses a black dot on the bottom of the camera body, in the tripod socket, as in the picture below, you should be aware that the camera has already been examined and measures have been taken so as to prevent this issue from occurring.
Nikon's notification with regard to this problem can be found in its entirety on Nikon's official website.