Enforced Method Acting: An unintentional example; Kristen Bell found out Veronica Mars had been cancelled the day before she shot the scene where Sarah found out her show was cancelled.
Also the whole scene where Jason Segel and Russel Brand poke fun at the stupid horror movie premise of killer cellphones, not knowing that Kristen Bell (who's sitting right next to them) actually starred in the real-world horror movie Pulse which revolves around exactly that premise not a year before.
Hilarious in Hindsight: A Dracula musical was eventually produced and performed on Broadway. No puppets, unfortunately.
Old Shame: Jason Segel revealed in interviews that the Dracula puppet musical Peter is writing in the movie is, in fact, based on a real Dracula musical which he began writing in his youth, and the song he sings in the karaoke bar is a real song from that musical. He claims to have played a demo tape for Judd Apatow, whose only response was "You can never let anyone hear this tape." Listen for yourselves. The anecdote starts at 13:30.
Write What You Know: Jason based the premise of the movie on his own break up with Linda Cardellini. The scene where he drops the towel and is naked is based on what really happened to Jason when Linda broke up with him.