Beside the Dying Fire - S2-E13
Continuity mistake: When the farm is overrun, the tires on Daryl's motorcycle are offroad tires. Later in the episode however, these tires change magically to smoother street tires. (00:15:35 - 00:28:40)
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: The walkers could have knocked them down more than once. There may have been another wave that knocked down the fence earlier and they're going to put it up again soon.
Captain Defenestrator