This "What If" scenario takes place just prior to the events we've now come to know as "Twilight."
Jacob Black is out hiking and exploring in the woods one day. Making his way along a narrow ledge, a rock gives way under his feet causing him to fall over a small cliff. Falling awkwardly for ten feet or more, his right knee hits first taking the full force of his weight impacting the ground. His femur shatters, severing his femoral artery, and jutting out through the skin of his thigh. Blood gushes from the wound, Jacob will be dead within minutes. Fate intervenes, and as it happens, Carlisle and Esme Cullen are out hunting together that day. They smell human blood, and of course, they decide to investigate. As they come upon Jacob, he is nearly unconscious, fading fast, and there is no time to get him to the hospital where he might be saved through medical means. Carlisle is horrified by the prospect of the decision facing him, and several thoughts race through his advanced intellect at unimaginable speeds. According to the treaty, he is bound to let Jacob die right in front of him, ignoring his drive to preserve life. If he does change Jacob, it would violate the treaty, but Jacob would live, and since he is one of their own, the Quileuttes would have no choice but to accept him... wouldn't they? Would they forgive Carlisle the violation since it was only to save Jacob's life? Would this bring a new level of peace and understanding between the Cullens and Quileuttes, or cause an all-out war? Only a microsecond has passed, and just then Esme gently places her hand on Carlisle's shoulder, speaking in her ever calming tone. "You can weigh the choices all you want, but you know what has to be done. It isn't in your nature not to help someone, the consequences will be whatever they will be, and we'll deal with them when the time comes."
Ladies and gentlemen, I now present to you Jacob Black: vampire.
This scenario takes place while Jacob is still a human, before the onset of the fever which leads to his transformation into a shape-shifter. Does he then still become a shape-shifter? If so, does this transformation kill him since vampire venom is poisonous to shape-shifters? Does he become some sort of vampire/shape-shifter hybrid?