Revealing mistake: Carl is walking toward a walker embedded to a tree. In the background there's a female walker wearing a distressed white dress with long black hair and a bear trap caught on her leg, standing in the back waiting for her scene to walk out. (00:22:30)
Visible crew/equipment: Visible crew behind the tree towards which Carl walks back after replying to Hershel's "it was so peaceful" comment. (00:22:20)
Answer: Walkers are attracted to noise, they can tell who's dead and who isn't dead, in S1 when Rick and Glenn cover themselves in walkers' guts they're able to get far by walking through the herd of walkers because they smell like they're dead, but as soon as the guts get washed off by the rain the walkers immediately attack them, if you draw attention to yourself they will attack you which we've seen that many times throughout the series that it's not always successful. Whether you're covered in guts hiding under dead corpses or under a car.