That some zombies continue to attack and damage you seconds after having their heads shot off and others drop instantly? The worst is when you shoot a zombies head of and then get downed by it. Is this by design or a flaw in the game?
No the worst is when you kill the zombie and pick up the powerup that he dropped but somehow get downed by the his arm brushing you as his lifeless body hits the ground. They aren't always harmless until they go all the way down.
PS: Never trust a nuke. I've been killed several seconds after setting one off while waiting on the Zombies to die.
Its when your gun looses power as rounds go buy. Its powerful enough to take there heads off but not powerful enough to finish them off. All you have to do is shoot it one more time or you can knife it and still get max knife points for it.