http://forums.steampowered.com/forums/showthread.php?t=746172 The Helicopter Pilot, listen to his sound files and you will discover something. Future Campaigns?
The helicopter pilot was the one who was supposed to drop you off on the roof, he also was supposed to be infected and die while flying his plane and lead you to the next level.
So lots of custom campaigns. Within 5 months there'll be about 30. http://www.l4dmaps.com/files.php?cat=3
Just somethings custom doesn't mean it can't be flawless. The original scenarios still have tons of bugs and were even buggier before the game was released.