Originally Posted by Rowan
....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).
When threadless headsets were first on the market, broken (plastic) top caps were common as even shop personnel weren't real familiar with the proper adjustment sequence. I remember aluminum top caps being offered as aftermarket accessories to reduce the breakage.