I heard a Master London Baker being interviewed a couple of weeks ago and he said that if you buy a dozen bread rolls from a bakery you should, by rights, get 13. He explained why it came into being, to do with weights a long time ago and the 'extra one' was to ensure you got the right amount. Has anyone asked for a dozen and got 13? I never buy that much bread in one go!!
It used to be quite common amongst local bakers round my way, but the only time I've seen this more recently has been the occasional promotion in supermarkets offering a 'Baker's Dozen' on pre-packaged rolls.