Factual error: When Rick has given Andrea his revolver to end her life, he leaves the room with the others to leave Andrea and Michonne alone. The next thing you see is Rick, Tyrese and Daryl waiting outside the room. You hear the gun go off and then you hear a tinkle as a shell casing hits the floor (noticeably "lighter" than the sound the revolver would make when dropped). Revolvers do not eject a shell casing.
Other mistake: When Martinez shoots the grenade launcher for a second time the cross hairs are on the concrete wall of the guard tower yet the explosion does nothing to the concrete and destroys the top of The Tower.
Continuity mistake: When Andrea is attempting to retrieve the pliers on the floor she picks them up between her toes with the head facing up. In the next show the pliers can be seen with the head facing down when she grabs them with her hand.
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.