Other mistake: Earlier in the season Father Gabriel Stokes burns his clerical collar, but in the end of this episode he is wearing it again. Especially doesn't make sense, because he said he gave up being a pastor.
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