Continuity mistake: When Rick, Carl, and Michonne are leaving King County (Rick's home), they must travel around a wreck involving two cars and an overturned RV. They drive along one side of the road. When the camera angle changes, they are on the opposite side of the road going the other way. (00:41:30)
Continuity mistake: When Rick, Michonne, and Carl go out on a run, in the first shot, Rick starts to roll the window down so he can shoot a walker. The next shot the window is up, and he rolls it down again. (00:02:00)
Continuity mistake: When Carl shoots the masked man who is found to be wearing body armor, Rick pulls up the armor to find the bruised ribs of a Caucasian male. When the assailant is unmasked however we see that the Caucasian male has magically transformed into an African American man. (00:11:20)
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