Revealing mistake: When the Governor is hunting Andrea, he smashes a few windows in the building where she's hiding. When he breaks the first two, there's a jet of steam or air, showing that it was broken a split second before he hit it with the shovel. (00:29:04)
Other mistake: In Woodbury, on the street, while Andrea talks to the Governor, we can see everybody wearing jackets and orange, red and yellow trees, showing that the scene was shot during fall. According to the beginning of the third season, they had just passed through a hard winter. So it should be spring or the beginning of summer.
Revealing mistake: After Andrea avoids the truck on the road, she is attacked in the woods by some walkers and pinned to a tree. After breaking free, she goes to stab a woman on the ground with her knife. But as she goes to strike, you can see the actress slowing her strike down "pulling the punch" and gently resting the blade on the walker's head instead of actually striking. (00:20:41)
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.