Hostiles and Calamities - S7-E11
Revealing mistake: When Frankie and Tanya visit Eugene while he's playing the video game, he refuses to give them the pills he made. They call him a coward then leave, and in the next closeup of Eugene's video game screen we can see his reflection, but the problem is his reflection is not reversed as it should be (note his shirt buttons, etc). (00:44:10)
The Day Will Come When You Won't Be - S7-E1
Revealing mistake: When they pick up Glenn's body, for a brief moment, you can see his head is fully intact.
Revealing mistake: When the two cars drive down the freeway with the cable in-between them, there is no blood or body parts on the cars at the end despite the carnage, suggesting CGI effects.
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