I suspect an isolated incident.
I've reached the same conclusion after further correspondence with Robin Thorn. He asked me to check the lengths of the spare spokes which were supplied with my 650B Mercury. These proved to be one each of 274mm and 276mm and five of 238mm. The longer spokes are for the different sides of the front wheel (and effectively confirm that different lengths were used for the two sides) and the shorter spokes are for the Rohloff hub (which come in packs of 37).
I suspect the failure occurred when I failed to avoid a particularly nasty pothole when going down a hill and am happy that the nipple proved to be the weakest link. It was a few days before I noticed that the front wheel was slightly warped so it wasn't a ride stopper. I've now got the wheel running very close to true by using the upside down bike as a truing stand. I'll be getting a few more spare nipples just in case there's another failure in the future. The problem with these small components is trying to remember where I put them!
