When you attempt to shift-click it in order to remove it from yourself and put it into the inventory, and there is not enough space, the backpack will end up in the now-nonexistant additional space it previously provided. Often this will happen: http://scr.hu/38wr/sc1v4 , and if not that, then it will be completely devoured. Then you'll have to get ANOTHER backpack to retrieve the old backpack from itself.
Mind-boggling.