Internment - S4-E5
Continuity mistake: When Rick gets out of his car and talks to Maggie inside the prison gates. The car door opens and shuts throughout the conversation.
Internment - S4-E5
Continuity mistake: In the previous episode, Rick and Carol are shown hunting through houses for household drugs to fight the flu. They even make a mention that most of what they find expired a year ago, but to bring it back to Hershel anyway, and let him decide about it. But in this episode, Rick returns, talks to Hershel, and no mention is made about him bringing medicine. Considering how important it was to fight the flu with any means possible, Rick would not have let Carol drive off with the potentially life-saving drugs.
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