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 item 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.