Continuity mistake: When Aaron and Daryl are stuck in the van surrounded by walkers, he lights up a cigarette with a yellow filter and it changes to a white filter as he starts smoking it.
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)
Continuity mistake: Gabriel burns his collar while eating at the side of the road, however when trying to stop the barn doors opening in The Storm it is back again.
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: During a fight with Gregory, Maggie's right cheek gets cut and blood runs down her neck, but the next day talking to Rick her right cheek shows no sign of ever being cut.
Continuity mistake: Maggie comes over to Rick's porch to chat about the group not being whole. When she arrives the sun is going down. She even says something about putting kids to bed. Near the end of the scene there is a closer shot of her and it is afternoon.
Continuity mistake: When Rick and Aaron are loading up the supplies from the houseboat, they put a container with a red can in straight and flush with the other container, so that the lid sticking up is to the left. In the interior shot of the van, the container is twisted sideways and pushed back with the lid to the front.
Continuity mistake: At the end 3 men go to rush Rick - the guy on the right is Glenn. All three men are standing shoulder to shoulder when Rick pulls his gun, and Glenn has disappeared. Cut back to Rick and a wide shot - no Glenn. Cut to a close up of the group and Glenn's back, but there is a big gap between him and the other two men. This scene is continuous - he hasn't got time to move.
Continuity mistake: Daryl is caught in a classroom and fights one of the Pope's men. He has him in a corner by the door where a walker comes. The walker gets killed by people on The Other Side of the door and it noticeably splatters on the glass. Two people break the door down and get inside. The splatter disappears on the glass.
Continuity mistake: When Carl gets shot, and we see him in the ground, not only does the blood spot on his short change in the next shot of him on the ground, but he's in a completely different shirt. It's still a cameo shirt, but the cameo pattern is different.
Continuity mistake: Towards the end when Rick and Shane are leaning up against the car, Rick says it can't be that easy killing anyone, you know that. Shane is looking ahead with his head angled down in that shot, but when the camera cuts to a different angle, Shane is holding his head up right. (00:40:15)
Continuity mistake: At the crossroads, after Rick starts to walk away from Shane, Shane starts to tell the story of what happened with the outbreak. Rick pauses and is looking off to his right as he is in the blurry foreground with Shane talking behind him, looking towards the left of camera. The angle cuts to a view from behind Rick, and suddenly Rick has his head turned to his left now, but also towards the left of camera but from this angle. (00:16:43)
Continuity mistake: Rick tells Hershel to cover Glenn. And as he's telling him to see if he can make it to his car, Hershel isn't looking at Rick, but straight ahead deeper into the bar. When the camera cuts to a different angle, suddenly Hershel is looking right at Rick. (00:13:55)
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.
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