[ratter]

Wrongly adjusted limit screws would have shown up before this I would think, basically when the biggest gear was selected for the first time.

I had a similar thing happen to me once on my old mountain bike, but it was caused by a chain failure, a link opened up and caught on the rear derailleur, pulling it into the spokes and then grabbing and bending it more.

many people have a habit of lying there bike down or leaning it against a wall etc on the right side, this can bend the rear hanger and put the adjustment out.