Continuity mistake: When the Winnebago breaks down. Shane and T-Dog offer to drive ahead to check out what could possibly be a gas station. Shane tells Rick to hold the fort while they're gone. Meanwhile Rick is dealing with dying Jim. When discussing Jim's predicament, Shane is there, he hasn't gone anywhere. Then when they put Jim under the tree T-Dog is there too. Then they all drive off like the breakdown never happened. Nothing is mentioned about whether Shane and T-Dog found a gas station and if they did, it was very quickly sorted out - two minutes tops. (00:31:00)
Audio problem: In the scene when Andrea is mourning the death of her sister, Rick walks up to "tell her how it is." Andrea points a gun at Rick with just her right hand. When the gun is coming up to Rick we hear the slide being cocked, this can only be done with two hands. (00:03:35)
Factual error: One of the machine guns set up to defend the CDC in Atlanta is a Soviet DShK machine gun. This weapon would not be in use by American troops in an American city.
Factual error: The microscope in the CDC lacks objective lenses. (00:36:27)
Answer: The walkers could have knocked them down more than once. There may have been another wave that knocked down the fence earlier and they're going to put it up again soon.
Captain Defenestrator