Be careful when you are downloading files from the internet even if you are using a mac. I downloaded a cool Christmas desktop background and applied it as my background and then suddenly my dock disappeared. My mac dock was missing. I tried re-starting the computer and checking the preferences but nothing seemed to work. Then I located the desktop background image file and moved it to the trash and then emptied the trash and my dock came back just as suddenly as it disappeared. Weird? Has this happened to anyone else?