it has only happened twice in 12000 miles. First after delivery from dealer (2009) and second, after my last chain adjustment (a couple of weeks back)

now it could have been I didn't finish it off properly (strong possibility!)

Also maybe it is my technique:

I tighten the adjuster nut till hand tight then holding it in place with a spanner, I torque the lock nut against it.

If I don't hold the adjuster nut, it will move tighter when torquing the locknut. Maybe I should let this happen (??)

I was wondering why yamaha only specifies the torque figure for the lock nut and not the adjuster nut (one pushing against the plate)