Kristen Bell injured her knee while filming a horse scene, which didn't make the final cut. In scenes towards the movie's ending, she walks with a slight limp.
The film is based on script-writer Jason Segel's experience breaking up with Linda Cardellini as well as three other breakups with unspecified women. Segel has said that the "naked breakup" did not involve Cardellini and that she was a great girlfriend.
Hawaii's state fish really is the Humuhumunukunukuapua'a. It is pronounced: "who-moo-who-moo-noo-koo-noo-koo-ah-pooah-ah".
When the film was first released, Universal promoted it by hiring skywriters to fly over cities where it was playing with messages like, "I hate you, Sarah Marshall!" Real-life women named Sarah Marshall got calls from their friends asking if they were having any trouble with their husbands or boyfriends. Some complained to the authorities, but were told that under the First Amendment there was nothing they could do to stop it.
An action figure of famous psychiatrist Carl Jung can be seen hanging from the side of Peter's refrigerator. The quote on the box reads, "Everything that irritates us about others can lead us to an understanding of ourselves."
Cynthia Nixon: in the background, when Jason Segel's character mocks Sexo en Nueva York (1998) while getting drunk at the bar.