Revealing mistake: As the first gunman is shot in the chest you can see the square patch from the blood pack. (00:11:20)
Revealing mistake: Near the end of the show, while Rick is still in Atlanta (he is on a secondary paced road), you can see vehicular traffic moving on a bridge in the background.
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