Left 4 Dead 2 "mutations." They promised fun tweaks to the gameplay to keep it interesting and fresh. In practice, however, they seem more like an excuse for Valve to make the game even harder. For instance, this week's mutation is "Hard Eight," which doubles the maximum number of active special Infected from four to eight. Eight.
What's that? You just killed a Boomer? Well, don't worry, because there are seven more standing right behind you.