Revealing mistake: After it starts raining there is a shot looking at the road in The Distance. Notice how the rain is only falling in a small area and stops just beyond where the group is at. The road is dry from there on out.
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.
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