Factual error: When Rick has given Andrea his revolver to end her life, he leaves the room with the others to leave Andrea and Michonne alone. The next thing you see is Rick, Tyrese and Daryl waiting outside the room. You hear the gun go off and then you hear a tinkle as a shell casing hits the floor (noticeably "lighter" than the sound the revolver would make when dropped). Revolvers do not eject a shell casing.
Other mistake: When Martinez shoots the grenade launcher for a second time the cross hairs are on the concrete wall of the guard tower yet the explosion does nothing to the concrete and destroys the top of The Tower.
Continuity mistake: When Andrea is attempting to retrieve the pliers on the floor she picks them up between her toes with the head facing up. In the next show the pliers can be seen with the head facing down when she grabs them with her hand.
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