I came across a bike whose owner was at a loss as to why the headset was always loose. This was after he had even taken it to a bike shop. On closer inspection, the following was found: (a) the steerer tube was too long or the spacers were too low so the top cap was pulling down only on the steerer tube; and (b) the shop had tightened down the top cap to take up the slack -- then had popped the recess in the cap for the bolt!!! Some star-fangled nuts are obviously tougher than others (or maybe the top caps are weaker).