Revealing mistake: When the two cars drive down the freeway with the cable in-between them, there is no blood or body parts on the cars at the end despite the carnage, suggesting CGI effects.
Beside the Dying Fire - S2-E13
Other mistake: While the farm is being overrun, Hershel is out front with his shotgun constantly firing. He never reloads and would never had the chance to stop and reload that weapon with so many zombies on him. His shotgun appears to have infinite ammo at this point.
Continuity mistake: When Carl shoots the masked man who is found to be wearing body armor, Rick pulls up the armor to find the bruised ribs of a Caucasian male. When the assailant is unmasked however we see that the Caucasian male has magically transformed into an African American man. (00:11:20)
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.