Beside the Dying Fire - S2-E13
Other mistake: While the farm is being overrun, Hershel is out front with his shotgun constantly firing. He never reloads and would never had the chance to stop and reload that weapon with so many zombies on him. His shotgun appears to have infinite ammo at this point.
Answer: In the season 2 finale we find out that everyone is infected therefore you turn no matter what happens to you (which proves that Dr Jenner was right, we see it happen when Rick stabs Shane).