Shortly after the DSLR hit the market, users reported annoying spots appearing in their images. Both D600s I’ve used have exhibited this problem, one more than the other. The problem has been variously described as being shutter lubricant (aka oil) splashing onto the imaging sensor, or dust.
Nikon worldwide, including Nikon Canada, says that if you send the camera in to the company, it will inspect, clean and replace the shutter assembly and related parts of your camera, free. This includes the cost of shipping your D600 to Nikon and its return to you.
Nikon will do this even if Nikon’s warranty has expired.
Before you rush to the post office, check out this link to Nikon Canada’s site for details on what you have to do, and how.