Other mistake: When Daryl is cutting open a walker in the woods to see what meat it ate while searching for Sophia he stabs the walker several times, but the knife is not bloody and it stays clean. (00:33:30)
Pretty Much Dead Already - S2-E7
Other mistake: As Daryl carries a saddle toward the barn, in the far shot, there is no one around. The shot then changes to him entering the barn carrying the saddle inside. Suddenly we now see Carol coming around the corner, several yards behind him. Where did she come from? Unless she was already inside next door and saw him enter, she shouldn't have been there.
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