In this movie, Jack Nicholson plays a notorious bachelor, who always "escaped the noose", while Diane Keaton plays a divorcée. In real-life, it's the other way around: Nicholson is a divorcée, while Keaton is a bachelorette.
First (albeit brief) full frontal nudity scene for Diane Keaton. Although highly publicized as her first actual nude scene, in fact, she first appeared topless (semi-nude, not fully nude) in Looking for Mr. Goodbar (1977).
When Erica kisses Harry in the scenes outside the restaurant in New York City, it was an improvisation by Diane Keaton. Writer and Director Nancy Meyers liked it so much, that she decided to use it in the final cut of the movie.
The American poster for this film featured large pictures of Jack Nicholson and Diane Keaton. In Japan, the poster also included a large picture of Keanu Reeves, presumably because of his popularity there at the time.
The DVD includes a deleted scene where Harry sings "La Vie En Rose" for Erica in a karaoke bar. Part of this scene (Harry and Erica dancing) was in one of the trailers. Jack Nicholson sings the same song during the end credits.
With this film, Sony became the first studio to release nine films that opened at number one at the box-office in a single year. They broke the record in 2006, with the release of Gridiron Gang (2006).
After Harry is forced to move into her beach house following his heart attack, Erica complains that she has become "a character in a Kaufman and Hart play!" This is a reference to The Man Who Came to Dinner (1942), a play by George S. Kaufman and Moss Hart, in which an obnoxious house guest, Sheridan Whiteside, is forced to move into a family home after breaking his hip, and drives his hosts crazy with his outrageous demands.
The trivia items below may give away important plot points.
After Erica and Harry have spent their first night in bed together and Harry is getting ready to go back to his room and they're saying their good-nights, the piano in the background score quietly but distinctly hits the first 5 notes of "Let's Fall In Love".
There are several moments in the latter half of the film when Erica and Harry are together, particularly the climactic bridge scene, that the background score seems to pick up some of the thematic motif of "Defending Your Life (1991)". This would be appropriate in the sense that "Defending Your Life" suggested that karma -- going back to Earth to try again or advancing to the next level -- is about living your life with the courage to reach for things that are important to you.