Factual error: Rosita picks up a spent cartridge fired from the desert eagle earlier in the episode, which is either a .357 Magnum, .44 Magnum, or .50 AE as those are the only chambers that the desert eagle is made in. She then asks Eugene to make her a bullet. In the following episode he does and she uses that bullet to fire a pot shot at Negan. But the casing that she picked up would not fit the 9mm Beretta she uses, nor could it be modified to do so.

The Walking Dead (2010)
1 factual error in season 7 - chronological order
Starring: Jon Bernthal, Sarah Wayne Callies, Steven Yeun, Andrew Lincoln
Genres: Horror
Other mistake: Earlier in the season Father Gabriel Stokes burns his clerical collar, but in the end of this episode he is wearing it again. Especially doesn't make sense, because he said he gave up being a pastor.
Rick Grimes: This is how we survive. We tell ourselves that we are the walking dead.
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)
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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