Revealing mistake: As the first gunman is shot in the chest you can see the square patch from the blood pack. (00:11:20)
Continuity mistake: When the stinger is deployed it covers the whole road but in the front long shot it doesn't. (00:09:05)
Continuity mistake: When driving to the pursuit, the cop in the passenger seat is putting his first glove on. Camera changes and now he has both gloves on. (00:08:25)
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