[asto]

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.

[Mr. Zero]

For L4D2, the developers notice this tend and added a cvar called z_reserved_wanderers.

The cvar allows you to define number of "reserved wanders" to make sure Survivors will always meet some wanders even if rushing. Through you might have to alter some more cvars in order to force the director to always spawn wanders.

Reserved wanders will be taken out of the total pool (z_common_limit) and the director will use those reserved wanders to "counter" rushers by forcing wanders to always spawn.
Though in Expert games, there is a wanders cooldown to prevent survivors from being too stress. That can be overrode with director_relax_max_flow_travel (set it to 250 or so, 3000 is default).

If you have a server that can handle it, I personally use z_common_limit 45 and z_reserved_wanderers 15 with director_relax_max_flow_travel 500.
That way Survivors that just pushes forward no matter the case, will always be met with half-a-horde, 15 commons.