Owners reported multiple granular dust spots appearing in images, which returned after cleaning. Nikon's remedy replaced the shutter and related parts, which identifies the shutter mechanism as the source of the debris.
Nikon acknowledged that the structure of a digital SLR makes it difficult to eliminate dust spots entirely, but that in some cases a large number of clearly visible spots could appear. Its remedy was inspection, cleaning and, where necessary, shutter replacement, with camera replacement where repeated servicing failed.
The D610 arrived roughly a year after the D600 with a revised shutter mechanism. A successor released that quickly, differing principally in the component implicated in the fault, is unusually direct evidence of a hardware problem being designed out rather than merely serviced.