30 Days Without an Accident - S4-E1
Continuity mistake: After Michonne gives Rick the shaver, he has his gloves in his right hand and the shaver in the left. As he walks up to the prison, he throws the shaver away on the ground. He continues up to where Carl is brushing the horse, still with his gloves in his right hand and left hand empty. He remains like this whilst talking to Carl, but as he pats the horse, then walks away, he has the shaver in his right hand and the gloves in his back right pocket. (00:11:44 - 00:12:14)
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