In the Trivia for the movie, it says Seth Rogen says that there is a secret celebrity cameo that last for just a split second, and that no one has guessed it yet.

I saw it tonight, and I thought I saw a crazy cameo, and thought no way. But now that I read this trivia, I wonder.

At the end when they run into Danny and his team of cannibal, they have a quick cut of two of the cannibals, and I swear that one of the looks like Bruce Springsteen.