Continuity mistake: At the crossroads, after Rick starts to walk away from Shane, Shane starts to tell the story of what happened with the outbreak. Rick pauses and is looking off to his right as he is in the blurry foreground with Shane talking behind him, looking towards the left of camera. The angle cuts to a view from behind Rick, and suddenly Rick has his head turned to his left now, but also towards the left of camera but from this angle. (00:16:43)
Continuity mistake: Towards the end when Rick and Shane are leaning up against the car, Rick says it can't be that easy killing anyone, you know that. Shane is looking ahead with his head angled down in that shot, but when the camera cuts to a different angle, Shane is holding his head up right. (00:40:15)
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