That all depends on how strong the dogs pack drive is. There is a similar situation with personal protection dogs, and even sometimes schutzhund dogs, where if the owner falls to the ground, the dog will change its attack from the decoy(or live example for real life situation) to the actual owner/handler. It is bizarre but it can happen. If this is the case the dog is trained to redirect its prey drive, but if the dogs pack drive is high enough, it is never an issue. But what Euro-monster said is 100% true and CAN happen, although it really depends on how strong of a grasp you have over your dogs mind and how well it respects the heirarchy, and also the pack drive as I stated. Every dog is different
What is Schutzhund? (from schaferhund.com)
Schutzhund is a German word meaning "protection dog". It refers to a sport that focuses on developing and evaluating those traits in dogs that make them even more useful and happier companions to their owners.
Schutzhund work concentrates on three parts. Many familiar with obedience work of the American Kennel Club's affiliates will recognize the first two parts, tracking and obedience. The Schutzhund standards for the third part, protection work, are similar to those for dogs in police work.
While dogs of other breeds are also admitted to Schutzhund trials, this breed evaluation test was developed specifically for the German Shepherd Dog. Schutzhund is intended to demonstrate the dog's intelligence and utility. As a working trial, Schutzhund measures the dog's mental stability, endurance, structural efficiencies, ability to scent, willingness to work, courage and trainability.
This working dog sport offers an opportunity for dog owners to train their dog and compete with each other for recognition of both the handler's ability to train and the dog's ability to perform as required. It is a sport enjoyed by persons of varied professions, who join together in a camaraderie born of their common interest in working with their dogs. Persons of all ages and conditions of life - even those with significant disabilities - enjoy Schutzhund as a sport. Often, it is a family sport.