I've seen a total of 2 sheer pins break in all of my experience with auto pilot servos. One was a pitch servo and one was a roll, and in different planes. I wouldn't be surprised if it was just a fluke occurrence, maybe the screw was over-tightened or something like that. Either way, it's an easy fix, and I'm personally glad they had the sheer screw, because if it wasn't there and there was a more serious problem (servo locking up or something like that), it'd be the end, very likely, for pilot and passengers.