Continuity mistake: Glenn demands that Abraham stop the truck so he and Tara can get off, the two male characters then have a scuffle. We can see Eugene in the background standing next to the truck facing right of the camera. When the camera cuts back to Eugene by himself, he's now facing the opposite direction. Also the items in the back of the truck move from left to right. If you look carefully, you can see that the entire image was flipped horizontally.
Continuity mistake: When Eugene sees the walkers come out of the woods, the road is wet. The rest of the scene the road is dry.
Continuity mistake: When Carl and Michonne are eating breakfast, and she is rolling up the sleeves to her new white shirt, it is untied. When the scene cuts to her sitting down the shirt is tied at the back. (00:03:20)
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.