In the last years, I encountered 3 problems which needed an intervention of Nikon Switzerland: (1) continuous generating of dust particles with a new D4s (a bit like the D600 “syndrome”); (2) Nikkor TC-20E III: index came loose; (3) Nikon D850: body was generating error codes, which could be fixed by dismounting and mounting the lens again and again.
Problems (2) and (3) were handled promptly; for (1), however, it needed some serious hand twisting before they recognized the problem, and replaced the whole shutter mechanism. What is painful with Nikon, is their NEVER ADMIT GUILT policy. Each time I had to investigate what has exactly been done. For the D4s dust problem, the only mention I had on the repair sheet was “sensor cleaned”; months later, I encountered fortuitously a Nikon technician who knew my case, and told me about the shutter replacement; for the D850: sole mention: “verified”. My NPS representative investigated the latter case, and told me that they had to fasten the screws of the mounting base.