Just before the shot in which Hritik uses his father's computer and accidentally communicates with the extra-terrestrials, Rekha warned Hritik 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.
In the scene where where the super six kids first see Jadoo, one of the kids (Hansika Motwani) is missing. Later in the song, the entire super six group is present, including Hansika Motwani who wasn't present earlier.
The whole movie is based upon sending various sounds in space. But the fact is sound cannot travel through space. Thus the signal sent cannot reach to the aliens.
When Sanjay Mehra first receives the extraterrestrial signal, the floppy drive doors on two of the three drives are open in one shot, then closed in the next shot.
Rohit Mehra: You are a teacher, who's responsible for thousands of students, sir. You ought to inspire them to move ahead in life. Computer did not make man, man made the computer. So what the brains can accomplish, your computers never can, sir.