1. Amitabh Bachchan took 16 days to record "Shava Shava".
2. Shah Rukh Khan is only nine years younger than Alok Nath, who plays the role of his father-in-law in the movie.
3. Abhishek Bachchan made a cameo in the movie, which eventually got cut on his request.
4. John Abraham was initially offered the role of Robbie, but declined it.
5. Aamir Khan hated the movie when he first watched it, and left as quickly as possible afterwards to avoid the cast and crew.
6. Amitabh Bachchan and Jaya Bachchan appeared together for the first time in 20 years in this movie. Before K3G, the last movie they did together was Silsila in 1981.
7. Shah Rukh's oldest son, Aryan, played little Rahul in the opening credits of the movie.
8. Jugal Hansraj was put in one scene of the movie after Hrithik's introduction because Karan Johar considers him a lucky mascot.
9. While most know that the character Yash Raichand was named after KJo's father, Yash Johar, a lesser known fact is that Raichand and Johar share the same birthday (Sept. 6).
10. Hrithik tried on a t-shirt that was too short for him, and loved the look so much that he actually cut the bottoms off the t-shirts he wore throughout the film.
11. Karan Johar was sued in a UK court for using "It's Raining Men" in Poo's introductory scene in the movie.
12. Kajol found it difficult to learn the Punjabi dialogue in the movie. Shah Rukh Khan would come on set and rattle off sentences in Punjabi just to annoy her.
13. The production team flew 50 dancers from London to Mumbai to shoot "You Are My Soniya".
14. "Suraj Hua Maddham" almost didn't make it into the movie. Sandesh Shandilya wanted to make a full album and not be a guest composer, but Johar requested for the song to be in the film. Impressed by his sincerity, Shandilya let him have the composition.
15. Poo's friend in the movie, Sonia, went on to become MTV VJ Ramona.
16. Waheeda Rehman almost played Amitabh Bachchan's mom in the movie, and even shot the title song before she dropped out of the project.
17. Karan Johar was so stressed during the shoot of "Bole Chudiyan" that he fainted due to dehydration.
18. He eventually ended up giving instructions on his walkie-talkie from a bed for the day.
19. Hrithik Roshan was cast even before Kaho Naa... Pyaar Hai was released, after Karan Johar met him at a Manish Malhotra store with his mom. Johar eventually saw a rough cut of the film and signed him on.
20. Karan Johar didn't want to cast anyone but BigB for the role of Yash Raichand. He had even said that he "wouldn't make the film if he refused".
21. The popular soundtrack influenced music outside India too. "Suraj Hua Maddham" was sampled in a song named "Temptation" by Omarion.
22. The super-sanskaari title song was copied into a popular song in Trinidad & Tobago called "Indrani", which is about a man asking his girlfriend why she's cheating on him.
23. And "Bole Chudiyan" was sampled in "Hello" by rapper D-Block.
24. When Rahul tells Naina that he's in love with someone else, the song that plays in the background is Tina's theme from Kuch Kuch Hota Hai, which was also played by Rani Mukerji.
25. They managed to shoot a sequence from "Deewana Hai Dekho" at the British Museum in London. The venue was extremely difficult to secure, and if it wasn't for Hrithik's stellar dancing abilities, they wouldn't have been able to complete the shoot.
26. Yash Johar claimed that the tagline of the movie – "It's all about loving your parents" – was the best gift anyone has ever given him.
27. The idea for the title of the movie came from Kabhi Kabhie (1976).
28. While Rahul doesn't say outright that he loves Anjali in the movie, there is a deleted scene from after "Yeh Ladki Hai Allah" where he admits his feelings.