Continuity mistake: Arriving at Alexandria, Rick is shown from an interior POV leaning into the back of the car and picking up Judith, who is mostly wrapped in a spotty shawl. As he is shown doing this from an exterior POV, the shawl is barely visible, bunched up behind her legs. (00:41:44)
Visible crew/equipment: After Aaron refuses to provide his group's location, Rick tells everyone to eat and get some rest. When Michonne follows Rick outside it cuts to the car, and just as the camera pans to the right the actor's green nylon T-marker is visible on the ground, under the car door. (00:20:40)
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