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: On the roof all the survivors are discussing their escape plan while T-Dog is lying down injured by Merle. In the next scene people are wearing rain coats and covering their bodies with the walker's remains as Rick proposed. T-Dog has taken off his cap and looks fine. Rick gives him the handcuff keys to free Merle. In the scene after, while Rick and Glenn went out the streets, the others go back on the roof to watch them from above. Nobody is wearing a raincoat and also T-Dog is in the same condition as before, lying down with his cap on. (00:24:00 - 00:31:15)
Trivia: Eugene is looking for his missing girlfriend, "Stephanie." Princess visits him. He shows her his pinboard with strings attaching clues. He's found out plumber Roman Calhoun, was the last person to see her and tracks him down to a dodgy outfit at "1634 Racine." This is the home address in "The Untouchables" (1987 film about the capture and imprisonment of Al Capone, starring Kevin Costner & Andy Garcia), of Jimmy the good Irish cop, played by Sean Connery. (00:16:44)
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