Visible crew/equipment: When Rick hugs his family, a crew member can be seen filming over the red car. (00:10:40)
Revealing mistake: As the first gunman is shot in the chest you can see the square patch from the blood pack. (00:11:20)
Continuity mistake: When driving to the pursuit, the cop in the passenger seat is putting his first glove on. Camera changes and now he has both gloves on. (00:08:25)
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.
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.
Continuity mistake: Merle is handcuffed to a pipe and tries to seduce Andrea, saying they are going to die anyways. When she says, she'd rather die, his hands are apart closer to his sides, then when it cuts to a close up of him, suddenly he's got his hands together with one finger pointed at the other hand. (00:16:33)
Continuity mistake: When the stinger is deployed it covers the whole road but in the front long shot it doesn't. (00:09:05)
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