Kristen Bell injured her knee whilst filming one of the horse scenes for the movie, which didn't end up making it into the final cut. However, she can still be seen walking with a light limp in scenes towards the end of the movie.
Screenwriter and star Jason Segel told New York Times interviewer Dave Itzkoff that both the naked breakup and Dracula puppet musical scenes were drawn from his real life experiences. In the article, Segel admitted that he really did once have a girlfriend who broke up with him while he was completely naked (although rather than being devastated during it, he thought to himself, "This is hilarious. I cannot wait for her to leave so I can write this down.") And before he was a successful actor, Segel tried to write a musical adaptation of "Dracula" for puppets.
Based upon script-writer Jason Segel's experience of dealing with his break-up from Linda Cardellini, as well as three other breakups (with unspecified women) besides Cardellini. Segal has stated that the 'naked breakup' did not involve Cardellini.
The T-Shirt that promotes "Norman's Rare Guitars", worn by Jason Segel's songwriter character in many Hawaiian scenes, is from a real store frequented by rock stars and musicians in Tarzana, California.
In at least two scenes in the film, a poster for the San Diego-based band The Rugburns can be seen hanging on the wall behind Peter when he is playing his piano. The Rugburns and their lead singer/songwriter Steve Poltz are known for their humorous and self-deprecating songs, much like the song Peter sings upon returning from Hawaii about needing to see a psychiatrist.
During the montage of Peter (Segel) and Sarah (Bell) on the different red carpets, one of the photographers shouts "André the Giant, out of the shot please!" In the 2009 film I Love You, Man (2009) Segel's character quotes André the Giant's line from The Princess Bride (1987) - "Anybody want a peanut?". In an episode of Saturday Night Live Segel even imitates André the Giant completely in the segment 'Andre the Giant Chooses An Ice Cream Flavor.'
The trivia item below may give away important plot points.
In a panel at San Diego Comic-Con on July 19, 2013, Kristen Bell told the audience that she learned Veronica Mars (2004) was canceled the day before she shot the scene in Forgetting Sarah Marshall where her character learns her show was canceled.