1. Karan Johar had the idea for the film floating around in his head for a while before he decided to hire Nikhil Advani to direct it.
2. Later, in his book, An Unsuitable Boy, he admitted that he regretted handing over the reins of his story to someone else.
3. Preity got injured a day before shooting the opening scene where she goes for a run, so they got a body double for the scene instead.
4. This movie was initially supposed to be called “Kabhi Alvida Naa Kehna” but after Johar heard a song titled “Aisa Milan Kal Ho Naa Ho” from the Kajol-Saif starrer Hamesha, he changed his mind.
5. They actually bought the rights to redo “Pretty Woman” for the film after they got into trouble for using “It’s Raining Men” in K3G.
6. Preity Zinta designed clothes for the dolls that Gia plays with to make them look more Indian.
7. The sad version of “Kal Ho Naa Ho” was sampled in a song named “Babylon Zoo” by Congolese artist Youssoupha.
8. Preity Zinta genuinely thought that the famous red dress in the movie was jinxed because every time she wore something would go wrong. Either the rain machine would stop working, or the shoot would get cancelled, or something else would go wrong.
9. Abhishek Bachchan and Vivek Oberoi were initially offered the role of Rohit, which was eventually played by Saif Ali Khan.
10. The man shown as Naina’s father at the beginning of the film is actually the film’s cinematographer, Anil Mehta.
11. Advani was known for being really “loud” and for "yelling at people" on the sets.
12. Kareena Kapoor declined the role of Naina because she wanted more money. This resulted in a prolonged spat between her and Karan Johar. She even admitted on Koffee With Karan that she regretted “missing out on the role of a lifetime”.
13. The tune for “Kal Ho Naa Ho” was composed at German Bakery in Pune by Loy Mendonsa, after Nikhil Advani told him he wanted a tune like “My Heart Will Go On” from Titanic.
14. In one of the scenes that Gia was supposed to cry in, assistant director Tarun Mansukhani yelled at the little girl so hard that she actually started crying.
15. Rani Mukerji was also hurt that she wasn’t offered the role of Naina, but Karan made up by offering her an appearance in “Maahi Ve”.
16. Salman Khan allegedly rejected Saif’s role because he didn’t want to play second fiddle to Shah Rukh Khan.
17. In fact, Saif agreed to do the role because he regretted missing out on Salman’s role in Kuch Kuch Hota Hai.
18. During the making of “Kuch Toh Hua Hai”, Preity Zinta fell before she could do the (now) iconic double clap.
19. Jaya Bachchan was literally cast because Karan Johar couldn’t visualise anyone else in the role of a mother than her.
20. It was apparently a nightmare to shoot “It’s The Time To Disco” because of all the fog used. The set got incredibly hot, making it near impossible to film, and the stars would change into other clothes between shots.
21. Karan Johar cut ties with composer duo Jatin-Lalit after they publicly stated their unhappiness over Johar using multiple composers for K3G. That was when he hired Shankar-Ehsaan-Loy.
22. Four days after the shooting began, Shah Rukh fell ill and was almost about to quit the movie. But Nikhil Advani convinced Karan to wait for him to recover before restarting the shooting again.
23. Karan Johar and Farah Khan make an appearance in the movie too, at Jenny’s café.
24. In a deleted scene, we see that Naina also used to read to the elderly in her free time.
25. Uday Chopra also briefly appears in the movie during the “Chhe Din, Ladki In” sequence.
26. Shah Rukh Khan’s kids have apparently never seen the actual ending of the movie. Karan Johar was kind enough to make a special edit of the film for them where the story ends before Aman dies.
27. During the shooting of “Pretty Woman”, Shah Rukh Khan told the dancers that the girl who played Gia (Jhanak Shukla) was “a big star back home and a robot” because she was the lead on a Star Plus show Karishma Kaa Karishma.
28. The North East Blackout of 2003 happened while they were still filming the movie, which threw the shoot schedule off for two days. At some point, they didn’t even have food on set.