At a RAA a couple of years ago we found two Rockets with incorrectly installed rear brakes. The bolt that stops the callipers from spinning round was not bolted into the hole. Fortunately both of them were done in such a way that the callipers would not rotate forwards, but if the rider were trying to use the brake to stop the bike rolling backwards down a hill then it could have got very nasty very quickly. IIRC both bikes had been incorrectly done by dealers.