Summary
from IMDB
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."
Following the destruction of the World Trade Center on 11 September 2001, images of the World Trade Center towers were digitally removed from all skyline shots of New York City.
The book that Jonathan searches for, "Love in the Time of Cholera", is also mentioned by John Cusack's character in High Fidelity (2000) in a list of books he's read.
On a subway staircase behind Jon, an ad for a play called "Six Degrees of Separation" can be seen. "Six Degrees of Seperation" is a theory which claims that any two given people on the planet are linked by a chain of only six mutual acquaintances.
Jonathan (John Cusack) tells Sara about the constellation Cassiopeia. In the movie The Sure Thing (1985), Cusack's character also tells the love interest (Alison) about Cassiopeia.