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: 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.
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.
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