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.
Continuity mistake: When Carl shoots the masked man who is found to be wearing body armor, Rick pulls up the armor to find the bruised ribs of a Caucasian male. When the assailant is unmasked however we see that the Caucasian male has magically transformed into an African American man. (00:11:20)
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