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.
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 five year gap between the initial meeting of the characters, and their reunion.
Kate and Johns characters are engaged to actors John Corbett and Bridget Moynahan. These actors once again play "the jilted lover" roles on Sex and the City. John Corbett's character was engaged to Sarah Jessica Parker (Carrie Bradshaw) and Bridget Moynahan was married to Chris Noths character Big. Their characters are again not "the right one" because Carrie and Big get together. So just like in Serendipity their characters get dumped so their fiancés can be with other people.
In this film, John Corbett (Lars) and Bridget Moynahan (Halley) are engaged and later dumped by characters who then end up together. The same thing happens to Corbett and Moynahan in the TV series Sex and the City (1998) where Corbett played the fiancé of lead character Carrie Bradshaw and Moynahan played the wife of Carrie's main love interest in the series, Mr Big.