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.
The Walking Dead (2010)
1 other mistake in Welcome to the Tombs
Starring: Jon Bernthal, Sarah Wayne Callies, Steven Yeun, Andrew Lincoln
Genres: Horror
Continuity mistake: When Daryl is driving the hospital van, it shows that he is going to land upside down, on the vehicle's roof, when he drives off the parking garage. In the next scene, the vehicle lands on its wheels, as if it has done a complete front flip. (00:30:00)
Indifference - S4-E4
Trivia: When Michonne and Daryl are walking toward the gas station sign, above the word diesel it spells out HELL in upside down numbers. (00:08:59)
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