Continuity mistake: At the end 3 men go to rush Rick - the guy on the right is Glenn. All three men are standing shoulder to shoulder when Rick pulls his gun, and Glenn has disappeared. Cut back to Rick and a wide shot - no Glenn. Cut to a close up of the group and Glenn's back, but there is a big gap between him and the other two men. This scene is continuous - he hasn't got time to move.
Continuity mistake: Sasha is pinned by the walker, but saved by Michonne. As she is standing up, the leaves in her hair disappear between shots.
Continuity mistake: At the end of the previous episode, Daryl leaves on a motorbike with a car following. In this episode Daryl's still out but no bike - he's in a car. It's a continuous trip with the bike he built at the compound.
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