Revealing mistake: When Rick checks his revolver before holstering it, there are no primers in his cartridges.
Continuity mistake: When Aaron and Daryl are stuck in the van surrounded by walkers, he lights up a cigarette with a yellow filter and it changes to a white filter as he starts smoking it.
Other mistake: When Gabriel is out by himself on the road and kills two walkers, he is wearing a white shirt. Blood is shown spattering everywhere, but his shirt remains spotless. (00:37:00 - 00:39:00)
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