Middle gear worries

[Mike F]

Hi All
I've just finished fixing my mate's xs11. He had been going down the motorway when the drive train started making an awful noise - can't reproduce what he actually said, my wife's in the same room!
On rotating the back wheel, I found that there was a tight spot so I undid the 4 bolts fastening the driveshaft to the middle gear. The tight spot had gone so I took the middle gear out and here's what I found.
The nut holding the coupling to the middle gear had sheared. The bike has only done around 15,000 miles and the middle gear has never been touched apart from oil changes. All I can think of is that it was overtightened during manufacture Has anybody out there any other theories?

[xssiveone]

Mike, I don't have any theorys on the cause, but I DO have one that looks exactly like it. It came with the boxes that contained my XS and I always assumed that the PO attempted to remove the nut without the aid of an impact wrench.