During filming of this movie in NYC, the following note was posted on a production trailer: "Yes, we are filming a movie. It is called 'Serendipity'. It stars John Cusack and some girl from Britain that you wouldn't know - even if we told you her name. We will be shooting tonight until midmorning. Please don't ask the crew any questions. Even if they don't look like they are working, they are thinking."
Jonathan tells Sara that he can make out the constellation Cassiopeia from the freckles on her arm. In the movie The Sure Thing (1985), John Cusack's character points out the constellation Cassiopeia to Alison, his love interest.
Serendipity mentions specifically or contains references to the following movies: Cool Hand Luke, The Godfather pts 1 & 2, Bridges of Madison County, The Wizard of Oz (pull back the curtain), Peter Pan (Never Never Land), and Star Wars (Alec Guinness / Obi-Wan Kenobi).
This movie has something in common with the t.v. series Sex and the City... Bridget Moynahan and John Corbit play the characters who have claims on the main characters in the movie and the show. Halley/Natasha is engaged/married to Johnathan/Big. Lars/Aiden is engaged to/dating Sarah/Carrie. Chris Noth also makes a cameo during Jonathan and Halley's engagement party.
The trivia items below may give away important plot points.
A gap of twelve-to-fifteen years was originally intended for the time between the Jonathan and Sara's first meeting and eventual reunion, and the characters were initially written to be in their early twenties. Audiences felt that both Kate Beckinsale and John Cusack were not 'fresh faced' enough to pull off early twenties in the scenes and that the time line had been too poorly established to tell that the characters met in the late 1980s. Eventually, the film was edited and establishes a seven year (according to when she lived with Minion) gap between the initial meeting of the characters, and their reunion.