1″ diameter, found objects

A generous friend who knows I like little things stopped by this week with two boxes of black gears. Beautifully milled and with good weight they look nice, but apparently their tolerances were a bit off and they failed some QA tests.  The gears got me thinking about tolerances. Just how much can things be wrong and still function?  How do we decide when things have gotten bad enough to say enough is enough? One more school shooting added to a long list of others and still, all we do is sigh.  I think we have to adjust our tolerances.