Left 4 Dead 2 Easter Egg - Common Enemy

In The Passing; 2: Underground, there is a building that is about three stories high after you get in the green pool and go through the tube. On the ground floor, there's a room where a bunch of refugees were hiding. There's a pool table in the middle of the room, and writings on the wall.

There's one wtiting on the wall in particular that's the actual easter egg: Need an M60? Call Crazy Dave.

Crazy Dave is the weird neighbor in Plants vs. Zombies who sells you stuff to help fight off zombies.

Read the other writings on the wall. Some of them are very entertaining