If you look closely, you can see all sorts of dirt on the image. (There's a very large noticable piece in the lower right and some other spots elsewhere on the right half of the picture.) This picture was taken shortly after I bought a new F2.8 24-60mm lens from Sigma. (I damaged my original lens when I dropped it in Okinawa.) The new lens is quite heavy due to the large pieces of glass, and it uses a nice sturdy metal ring for connecting to the camera body. Unfortunately, the previous two lenses I used had a plastic mount, and over time bits of plastic apparantly wore off on the metal camera connector. Then, when I used this new (heavy) lens with a metal mount, I think it dislodged a bunch of the plastic shavings and they found their way onto the camera sensor.
Fortunately, the camera sensor is easy enough to clean-off. A little bit of sand paper and some elbow grease and I'm all set. (Just kidding - you should NOT touch the camera sensor! A simple hand blower is all that's need to get the dust off. The camera has a menu item that lets you raise the mirror for cleaning the sensor.)