The game spawns max 30 zombies in front of the survivors (they called wanderer zombies). The problem is that if the survivors avoid those zombies, the zombies will be BEHIND the rusher survivors and the game will not spawn zombies in front of those rusher survivors.
The 30 zombie limit is changeable with z_common_limit cvar, but this is not solution, because in that case same will happen. Just more zombies will be behind the survivors.
Same happens when a mob spawned behind the survivors, and the survivors can rush forward the map, because there are NOT any zombies in front of survivors (all zombies are behind them).
http://www.youtube.com/watch?v=TzZ4lGwcRac
http://www.youtube.com/watch?v=-ger0k7mJkw
The worst thing with this bug is that if a boomer has vomited someone, and ALL zombies (zombies also who are in front of the survivors) will attack the vomited survivor. Other survivors let him die and they can rush without any problem.
This bug is very annoying, ruins the gameplay.