The D600 accumulated persistent dark spots on images, traced to debris associated with the shutter mechanism. Nikon inspected, cleaned and replaced shutter assemblies free of charge regardless of warranty, and in cases where repeated servicing did not resolve the problem, replaced the camera itself.
The programme closed in January 2020. The D610 followed roughly a year after the D600's launch with a revised shutter, which is unusually direct evidence of a hardware fault being designed out rather than merely serviced.