Continuity mistake: When Glenn wakes up in the prison you can see two chairs leaning against the walls behind him when he stands up. These are still present in the second shot, but when the camera moves back for the third shot they have mysteriously disappeared.
Continuity mistake: Glenn demands that Abraham stop the truck so he and Tara can get off, the two male characters then have a scuffle. We can see Eugene in the background standing next to the truck facing right of the camera. When the camera cuts back to Eugene by himself, he's now facing the opposite direction. Also the items in the back of the truck move from left to right. If you look carefully, you can see that the entire image was flipped horizontally.
Too Far Gone - S4-E8
Continuity mistake: When Hershel crawls away and the fight begins, in the long shots of the governor's men Hershel's body can not be seen next to the cars, this is from all angles.
Continuity mistake: When Mica and Lizzie are running from the walkers on the railroad tracks, they easily outrun them and gain several meters' distance in a few seconds. When they are running to the house, the walkers have suddenly caught up and are directly behind them.
Continuity mistake: When Glenn is with Tara and getting ready to move out, he puts on his helmet and you can see it is strapped on. When the shot changes, he swings the fence open and you can see his strap is lose. The camera angle changes again and the strap is back on again. (00:38:15)
Internment - S4-E5
Continuity mistake: When Rick gets out of his car and talks to Maggie inside the prison gates. The car door opens and shuts throughout the conversation.
Continuity mistake: In the flashback to the prison, Hershel says "we've been here two months." However, when they show Judith she is shown to be the same age she was when the prison fell (around 9-10 months) If they had only been there two months, and Lori delivered within the first week of being there, the baby should not be very old.
Too Far Gone - S4-E8
Continuity mistake: After The Governor shows up at the prison with his arsenal, Hershel and Michonne are taken out of the vehicles and placed kneeling on the grass side by side, about 1 arm's length apart from each other. But when Rick makes his way down to the fence to talk with The Governor, Hershel and Michonne are now about 4 arms' lengths away from each other. This goes back and forth for a few shots.
Continuity mistake: When Eugene sees the walkers come out of the woods, the road is wet. The rest of the scene the road is dry.
Continuity mistake: In the golf club Beth is wearing her new yellow t-shirt and white cardigan. Daryl hits walker blood all over her cardigan. In the next scene she takes the cardigan off and the t-shirt underneath (which was covered by the cardigan) has blood all over it. Back in the forest after this the shirt is clean again. (00:18:20 - 00:21:35)
Continuity mistake: Right at the end of this episode when there is another flashback to the prison, the baby is seen many times. Judith is asleep, then the camera angle changes and she is awake, it moves back to another angle and she is sleeping again. This is repeated several times.
30 Days Without an Accident - S4-E1
Continuity mistake: After Michonne gives Rick the shaver, he has his gloves in his right hand and the shaver in the left. As he walks up to the prison, he throws the shaver away on the ground. He continues up to where Carl is brushing the horse, still with his gloves in his right hand and left hand empty. He remains like this whilst talking to Carl, but as he pats the horse, then walks away, he has the shaver in his right hand and the gloves in his back right pocket. (00:11:44 - 00:12:14)
Continuity mistake: When Carl and Michonne are eating breakfast, and she is rolling up the sleeves to her new white shirt, it is untied. When the scene cuts to her sitting down the shirt is tied at the back. (00:03:20)
Continuity mistake: The message in chalk Carl writes on the door changes from what we see him do. For example, he writes something on the middle part of the door (which doesn't look like the word "shoe") and then doesn't write on the inset part of the door below that. When we see the message, the inset part now has some writing on it.
Too Far Gone - S4-E8
Continuity mistake: When the Governor and his group show up and bomb one of the guard towers to get Rick and his group's attention, when Rick first runs outside the guard tower on the far left is on fire, but when Rick walks down to the fence to talk to the Governor the guard tower in front by the front gate is the one on fire. And it switches back and forth between the two. (00:16:05 - 00:19:15)
Continuity mistake: When Glenn wakes up we see the door behind him is flush with the concrete, but when he goes inside you see stairs he had to descend to get to the door. There is also no scene in the entire prison episodes where an outer orange door is preceded by steps, they are all flush. Also If you look at the door behind him you see two folding chairs leaning up against the door. When he turns to open the door the chairs are gone.
Internment - S4-E5
Continuity mistake: In the previous episode, Rick and Carol are shown hunting through houses for household drugs to fight the flu. They even make a mention that most of what they find expired a year ago, but to bring it back to Hershel anyway, and let him decide about it. But in this episode, Rick returns, talks to Hershel, and no mention is made about him bringing medicine. Considering how important it was to fight the flu with any means possible, Rick would not have let Carol drive off with the potentially life-saving drugs.
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