If you're asking if anything "bad" will happen the answer is no. At least not to my knowledge. Depending on the lens you'll get some variation on what you see in your shots, a fuzzy area outside of the center of the shot where you run out of lens on the image sensor. I want to believe that if you were to shoot consecutive images, one with the camera set to FX and the other to DX that you would essentially get the exact same image quality if you cropped the FX image to match the DX. You may be able to squeeze a little more out of the lens in FX mode before getting distortion or vinetting, but probably not a lot. It just makes a little more work for you in post processing.