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.
The Day Will Come When You Won't Be - S7-E1
Continuity mistake: When Negan makes Rick get the axe from the top of the RV, Rick is fighting the Walkers as he heads for the RV door after picking up the axe. He slams a walker's head into the back of the RV, blood splattering everywhere. Next scene shows Negan and Rick driving back to the group, and the back of the RV is spotless.
The Day Will Come When You Won't Be - S7-E1
Other mistake: When Rick is on top of the RV, Negan peppers the roof with bullets from the inside, so that Rick has to run and jump onto the hanging walker. Negan's using a machine gun, so a lot of bullets, and the exit holes that are made, are visible on the roof. The softer material of the inside ceiling would have been really torn up, but when Rick gets back into the RV with Negan's axe, the ceiling is undamaged. (00:21:38 - 00:24:36)
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