Testing...
Not sure about the other guys, but here at the big Sky we test everything! New production runs gear boxes, blades, etc to the breaking point. Then we put it all on the new airframe, tie it down and run that to death too! When that all checks out we biuld 2 or 3 prototype aircraft and beat them up too! Needless to say, we try our best to root out all the issues before we sell our first production aircraft. Do things come up after the fact? of course, you can't possibly cover every scenario but we sure try!