Dust and oil. My first D600 went back after 400 shots, the next after just 80. Sensor was covered in oil/dust visible at F11 +. Brilliant camera that produces stunning images, but I just couldn't live with it. Found the extra £££ and got an 800 instead.
Any decent panographer would go through that many shots just setting up and playing with their new camera. It can take up to 10,000 before you really get to know your camera and everything becomes second nature.