Current best engineering practice is to design gear mechanisms that mesh in such a way that all parts of the system continually present different wear faces to other components. This ensures that all components wear evenly. Statistically with such a system there will come a time when all the weak spots in the components of such a gearbox will momentarily come into alignment. Similar to all the planets in the solar system lining up on one side of the sun. It happens very rarely, but when it does, that is the time that cracks might be initiated in highly stressed components.