Robert Downey Jr. was almost cast as Duckie when the ending had Andie getting together with Duckie. Per Molly Ringwald, this ending may have stuck if Downey won the role because he didn't give her the "brother vibe" Jon Cryer did.
The movie is dedicated to Alexa Kenin and Bruce Weintraub. Kenin, who played Jena, was murdered in New York just before the movie was released. Set decorator/production designer Weintraub (who had been Oscar-nominated for Prizzi's Honor (1985)) died of AIDS at age 33; Pretty in Pink (1986) was the last movie he worked on before his death.
When the ending was re-shot, all of the principal actors had to be called back. Andrew McCarthy had already lost a substantial amount of weight and shaved his head for a new role in a New York play called "The Boys of Winter". Although he wore an auburn wig, he's noticeably more gaunt in the re-shot scenes.
The Rave-Ups are the band playing in the club scene. Star Molly Ringwald was apparently a fan of their music and her sister had a child with one of the band members. Also, in Sixteen Candles (1984), "The Rave-Ups" is scrawled on the notebook Molly's character is carrying while walking down a corridor after study hall.
"If You Leave", performed by Orchestral Manoeuvres in the Dark, was not the original song Orchestral Manoeuvres in the Dark had composed for this movie. Their song "Goddess of Love" was the original composition, but it did not fit when the ending was changed.
The trivia items below may give away important plot points.
John Hughes was allegedly unhappy with the ending. He wanted Andie to get together with Duckie. But the film's ending of Andie getting together with Blaine was forced upon him by the studio. In retaliation, Hughes virtually made the same film all over again the following year with Some Kind of Wonderful (1987), a film about a similar love triangle. Hughes wanted Molly Ringwald to star in it as well, but she refused. Hughes took it personally, and effectively ended their working relationship. They never worked together again.
The original ending to this film depicted Duckie getting the girl. However, the test audiences said they would have preferred to see Blane win Andie's heart. Additionally, Molly Ringwald was sick during the filming of the ending and John Hughes wasn't satisfied with the editing. He was also concerned that audiences would take the original ending as a message that poor people and rich people don't belong together.
In 2010, for the 20th Anniversary of Entertainment Weekly Magazine, EW reunited Molly Ringwald, Jon Cryer and Annie Potts for a photo shoot and interview. The three discussed what they thought their characters lives were like after the movie ends. Ringwald said that she thought Andie and Blaine would have broken up shortly after the end of the film, but Andie and Duckie would have remained lifelong friends, and also that Duckie would have long since come out as gay.