Continuity mistake: At the end 3 men go to rush Rick - the guy on the right is Glenn. All three men are standing shoulder to shoulder when Rick pulls his gun, and Glenn has disappeared. Cut back to Rick and a wide shot - no Glenn. Cut to a close up of the group and Glenn's back, but there is a big gap between him and the other two men. This scene is continuous - he hasn't got time to move.
Other mistake: When Gabriel is out by himself on the road and kills two walkers, he is wearing a white shirt. Blood is shown spattering everywhere, but his shirt remains spotless. (00:37:00 - 00:39:00)
Revealing mistake: When Rick checks his revolver before holstering it, there are no primers in his cartridges.
Continuity mistake: When Aaron and Daryl are stuck in the van surrounded by walkers, he lights up a cigarette with a yellow filter and it changes to a white filter as he starts smoking it.
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.