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.
Visible crew/equipment: When Sam comes to visit Carol with items in his hand, he walks through the front door of the house. Carol shuts the door after she lets him inside. As she does this, you can see the reflection of a black sheet or tarp of some kind covering the door area where the camera reflection would be.
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