Left 4 Dead 2: [DLC] The Passing

[Skarro]

There is Graffiti on the wall from Frank West! The main character of Dead Rising! (Another zombie game) It reads..

"Otis, ran out of film, helicopter not coming, zombies too fast, won't get out alive, Frank West."

[Liraxus]

There actually are alot more easter eggs to this DLC, i can list them.

During the campaign, there is a rare chance you can see the Midnight Riders(A fake band in the game) tour bus rush through a street you can't get to, your character even comments on it.

Alongside the Frank West quote, there are alot more of them from other famous Zombie Movies/Games.

[NervousTyler]

Aaaand Ellis' friend Keith has also left his mark in various safe rooms, too!