1) Salman Khan was not the first choice to play the role of Kajol's fiance, Aman Mehra. Saif Ali Khan was supposed to be casted but dropped out and Salman Khan took his place. No harm done, right? *cue Saajan ji ghar aaye*
2) Kal Ho Na Ho director Nikhil Advani makes a guest appearance in 'The Neelam Show'. He is the guy who pleads his girl friend to leave him because he has found a new girl, who is played by Farah Khan!
3) The role of Tina was originally written for Karan Johar's BFF Twinkle Khanna. The character's name is also inspired from Twinkle's nickname, but Twinkle Khanna turned down the role.
4) Twinkle Khanna was not the only actress to turn down the role of Tina. Raveena Tandon, Aishwarya Rai Bachchan, Tabu, Urmila Matondkar and Karisma Kapoor turned down the role which ultimately went to Rani Mukherjee.
5) Kajol had a serious injury while shooting and actually lost consciousness for some time. During a bike riding sequence, Kajol lost balance and fell down badly, and cut her knee badly. You can see her losing control and almost falling off the bike in one of the scenes.
6) Rahul and Anjali used a trampoline to jump high during their first basketball match. The hoop was actually placed at a very low height and Shah Rukh Khan and Kajol jumped on a trampoline to shoot the scenes where they are jumping high. Guess he was a CHEATAAAAH after all!
7) The scene where Kajol's sari is flying in the air at the end of 'Ladki badi anjaani hai' was not originally in the script. Farah Khan added that scene to add dram to the song. Well, that scene did make Rahul realize that he was in love with Anjali, so we're not complaining.
8) Kajol, too, was not the first choice to play Anjali. Juhi Chawla turned down the role of Anjali, which was then offered to Kajol.
9) Karan Johar wanted Rani Mukherjee's voice to be dubbed. He did not think that Rani Mukherjee's husky voice would appeal to the audience, but it soon went on to become her trademark!
10) Farah Khan and Manish Malhotra had a cameo in the movie. They were a part of the people laughing at Kajol when she walks upto Shah Rukh Khan in the hideous orange- pink dress. Farah Khan had another cameo too, see #2.
11) Rani Mukherjee couldn't dance properly in the 'Koi Mil Gaya' song. Reportedly her dress was extremely tight and made it impossible for her to do her steps for the song!
(Facts courtesy IMDb)