When dragging an item that is in the Places bar onto itself, you get an annoying modal dialog "You cannot drop folders onto themselves". This should not happen, at least not as modal dialog.
If you e.g. have your Documents folder in the Places list and then drag the Documents folder from the view (i.e. you are currently in your home dir and then drag the folder from there and not the item on the Places list) onto the Documents folder in the Places list, then nothing happens, no dialog, no error, no context menu asking for moving/copying/linking.