1) they close an application's windows thinking the program has quit.
2) they'll download Firefox and then run the browser from the disk image and then not be able to throw away the disk image because the program is still running. See number 1.
3) .exe files lie scattered around the desktop from aborted downloads of Flash Player or some screensaver-cursor-spyware doodad.
4) untitled folder. untitled folder 2. untitled folder 3. untitled folder 4...
5) Using Safari's Google search field to get to hotmail.com.
6) Desktop Picture or Wallpaper is not a synonym for Screensaver.
I could go on, but you get the idea. What have you seen that are common mistakes made by people using Macs?
Note: Discussing common mistakes that people make doesn't have to be an exercise in meanness or criticism. In fact, I firmly believe that a problem can't be solved unless it's first discussed or pointed out.. Also, remember that comments are owned by the people who leave them, not by the author of this post. For more info, see comment #31.