[Jimmy Cao]

I ran into a similar failure mode with my P60 256:1. I think it resulted from applying a large amount of torque on the output shaft (trying to backdrive an arm) while the friction inside the gearbox caused the shaft to not rotate, thus causing the little pins to sheer the ringgear.

If you're in a pinch to repair it, I believe you can weld the ringgear on. I fortunately had a spare on hand, so I didn't need to use the one that was repaired yet, so I won't be able to testify as to whether or not it is more robust than the original.