Bolts usually break or threads get stripped from overtigthtening. Single bolts on most traditional quill stems use to be beefy and good for life if not overtighetned.Many actually would have been near impossible to break one. Although in the quest for lightness, I have seen single stem camp bolts that would seem to have been sized for water bottle holder use.So the answer is there is no answer. If you are heavy and stress stuff,don't overtighten and maybe avoid the lighter stuff witih marginal looking fasteners. Folks are having their share of problems with the new super light stuff more because of their own errors in installation, than due to any kind of problems with the product itself. The new stuff is often a good place for a torque wrench.There is always the possibility that the bolt itself was defective.