Continuity mistake: After Rick and Glenn cover themselves with guts, Rick tosses T-Dog the handcuff keys for Merle. T-Dog is at that moment in the rain coat and has on no hat, standing and appearing OK. The group then rushes up to the roof to watch Rick and Glenn on the street, and tell T-Dog to get back on the CB radio. Suddenly T-Dog is now leaned back sitting against the wall on the roof again with his hat and regular clothes acting all beat up again, and then shows Merle he has The Key.
Visible crew/equipment: When Rick hugs his family, a crew member can be seen filming over the red car. (00:10:40)
Continuity mistake: As the scene opens on the rooftop with Daryl, T-Dog, Rick, and Glenn observing the area that Merle had been handcuffed to the pipe, blood splatter magically appears around the severed hand when it wasn't there at the end of the previous episode. (00:05:10)
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)
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)
Factual error: The microscope in the CDC lacks objective lenses. (00:36:27)
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: At CDC headquarters, Dr. Jenner shows Rick and the group an MRI of an infected patient's brain activity as they die and subsequently resurrect. The patient is shot at point-blank range as the barrel of the gun can be seen in the MRI. This is impossible. MRI machines are highly magnetic and the gun would have been ripped from Dr. Jenner's hands with such force that not only would he not have been able to shoot, the MRI machine would have been destroyed.
Continuity mistake: At the end of the previous episode, the sun was setting but wasn't set fully yet. There was still a decent amount of light left outside as the door to the CDC was opened. At the start of this episode, as they are entering, it's already pitch black outside.
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