After his recovery, Mill Reef was trained for an autumn campaign and a return to the Prix de l'Arc de Triomphe in October. During a routine training gallop, however, he stumbled and shattered his foreleg. Charles Allen, a veterinary specialist, was flown in. Mill Reef's fracture was complicated. A triangularly shaped piece of bone about two and a half inches long was broken from the lower end of the cannon bone and was considerably displaced. The inner sesamoid bone was completely shattered, and the rim of the top of the main pastern bone was damaged. It seemed likely that the inner sesamoid bone was the first to break, with the result on the next step that the fetlock was not braced and the foot and the pastern were pointing outwards. The sheer pressure of the horse's weight caused the crumbling of the rim of the pastern bone and the breaking of the cannon bone.