Nikon has issued a service advisory regarding the Nikon D600
Nikon has issued a service advisory regarding the Nikon D600, especially with regard to the dust accumulation on the sensor. While understanding the fact that dust will occur over time, and steps may be taken to reduce the occurrence, the complete elimination of these dust spots may sometimes be difficult. It has come to our attention that, in some rare cases, they may be reflected noticeably in images and removal may be difficult using normal measures. As such Nikon is informing users of a service to reduce this issue.
As a first step, please follow the guidance from the D600 User’s Manual (pages 301-305) related to the Clean Image Sensor function and manual cleaning using a blower bulb. If these measures do not remove all dust particles and you are still experiencing problems, then please consult your nearest Nikon service center. The technicians will examine the camera thoroughly, and service it accordingly.