The mentally challenged character played by Hrithik wears spectacles. After going through various spectacles, the ones he finally used, were the same pair that he wore as ultra modern Raj in "Kaho Na Pyaar Hai".
Hrithik Roshan was the first actor to win both 'Best Actor Crictics' and 'Best Actor' at the Filmfare Awards for Koi... Mil Gaya. After him Amitabh Bachchan repeated the feat for 'Black' and Ranbir Kapoor for 'Rockstar'
Just before the shot in which Hrithik uses his father's computer and accidentally communicates with the extra-terrestrials, Rekha warned Hrithik that she would slap him hard. He thought she was joking, but when the camera rolled, Rekha actually slapped him hard, which left him shocked.
Hrithik Roshan was not able to reduce his weight enough for the role of a young adult with the mind of an 11-year-old because he was shooting other films at the same time. The problem was solved with baggy clothes that hid his real proportions and a special hairstyle that made his face look thinner.
The melody that the Earthlings use to communicate with the aliens is actually "Trans-Europe Express" by Kraftwerk. The song was first sampled in the remix "Koi Saaz To Chehro" from the album Garama Garam Asha Bhosle Volume 2. This was in 1991.
The makers of the movie had shot two different endings. In the alternate ending, the alien takes away all the powers with him. Rohit looks up to the sky and says thank you. But the director felt that the audiences would respond better to a happy ending. So the ending with Rohit getting back the powers was shot.
Rakesh Roshan wanted to cast Rekha as Hrithik's mother, but was apprehensive that she might refuse to play mother's role. He invited her home, on the pretext of seeking her opinion on the script. After listening to the script, Rekha was so excited, that she herself said, that she would like to play the Hrithik's mother in the film.
It is said that the director of the movie and father of Hrithik, Rakesh Roshan was inspired by some real life incidents from Hrithik's life. The scene in which the bullies break Rohit's cycle being one of many such scenes.