1. Viggo Mortensen auditioned for the role of Jake Ryan in Sixteen Candles, and Molly Ringwald wanted him for the role because during his audition, he kissed her. Michael Schoeffling (who landed the part) didn't.
2. Molly Ringwald, who plays Samantha, almost lost the part to Ally Sheedy, her future co-star in The Breakfast Club.
3. Jim Carrey auditioned for the part of Ted aka "The Geek."
4. When Molly Ringwald and Anthony Michael Hall first met, they hated each other. To remedy the problem, John Hughes took them record shopping and the pair bonded over similar taste in music.
5. As delicious as it looked, the cake in the end scene was made from cardboard.
6. The joke that Bender tells in the vent ("A naked blonde walks into a bar with a poodle under one arm...") actually has no punch line. According to Judd Nelson, he ad-libbed the dialogue.
7. Nicolas Cage was considered for the role of Bender, but production couldn't afford him.
8. John Hughes wrote the screenplay for The Breakfast Club in just two days.
9. The scene in the library where each character admits why he or she is in detention wasn't scripted, Hughes told the actors to improvise their lines.
10. Allison's "snow" dandruff was made using parmesan cheese.
11. There were plans for a Breakfast sequel, but the idea was squashed due to John Hughes' volatile relationship with Judd Nelson. Hughes went as far as to say he'd never work with the actor again.
12. For his role as Bender, Nelson went undercover at a local high school and convinced his "classmates" he was a legitimate student. He even bought them beer with his "fake" ID (he was 24 at the time).
13. The cast had to reshoot the end of Pretty in Pink, but Andrew McCarthy had already lost a ton of weight and shaved his head for a play, so he had to wear a wig while shooting.
14. Cameron's dad's car in Ferris Bueller’s Day Off wasn't a real Ferrari. Production didn't have the money to rent one, so they made three fakes, each with a fiberglass body.
15. To achieve the strung out look, Charlie Sheen didn't sleep for more than 48 hours prior to filming his scene as a drug addict at the police station.
16. The actors who played Ferris's parents, Cindy Pickett and Lyman Ward, got married in real life after filming the movie.
17. Johnny Depp, Rob Lowe, John Cusack, Jim Carrey, Tom Cruise, Robert Downey Fr., and Michael J. Fox were all considered for the role of Ferris.
18. The "Twist and Shout" group dance sequence was taken directly out of Michael Jackson's "Thriller" video.
19. While they played siblings on screen, Matthew Broderick and Jennifer Grey got engaged after filming this movie.
20. During her kissing scene with 14-year-old Ilan Mitchell-Smith in Weird Science, Kelly LeBrock said he got carried away and stuck his tongue down her throat. Afterwards she told him, "If you ever do that again I'm going to kick your ass!"
21. In an interview with Playboy, Robert Downey Jr. confessed to defecating in Kelly LeBrock's trailer (all because he was upset with John Hughes).
22. Home Alone was entered into The Guinness Book of World Records as the "Highest Box Office Gross - Comedy", accumulating $533,000,000 internationally.
23. In the scene where Harry bites Kevin's finger, Joe Pesci actually bit Macaulay Culkin, leaving a scar.
24. When Daniel Stern had the tarantula crawl on his face, he had to fake the scream (as to not upset the spider). He also said he would only do one take.
25. The photo Kevin finds of Buzz's girlfriend is really the art director's son.
All facts via IMDB.