Revealing mistake: As the first gunman is shot in the chest you can see the square patch from the blood pack. (00:11:20)
Revealing mistake: Near the end of the show, while Rick is still in Atlanta (he is on a secondary paced road), you can see vehicular traffic moving on a bridge in the background.
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.