Director Karan Johar never considered his good friend Rani Mukerji for the role of Tina. In fact, Rani read the script herself and went up to Karan saying, "I'm the only one able to do this role."
During a bike riding sequence in one song, Kajol lost control of the bike and cut her knee quite badly. You can just see her losing control before the camera cuts out the shot. Kajol hit the ground hard and actually lost consciousness for several minutes.
In one scene where Anjali and Rahul are talking at school, Anjali is leaning on a phone booth. When she moves away, the receiver falls, In the next shot, it's in its place again.
While dancing at the summer camp, Anjali is wearing a ring with AUM in her right hand's finger, however when she moves her hand on Rahul's face, she is wearing ring with a diamond. Then afterward, again the AUM ring appears.