There was news that Anushka Sharma auditioned for the role of Pia. However the audition video that was viral was of Munnabhai M.B.B.S. Also Kajol was supposed to play a role or a cameo in the movie but later turned down.
Aamir Khan became the first and only actor to kiss Kapoor sisters (Karishma in Raja Hindustani and Kareena in 3 Idiots).
Aamir Khan plays a college student in the film but in real life he is just five years younger than Boman Irani who plays the role of VIRUS in the film. Similarly, Ali Faizal who plays the character of Joy Lobo in the film is shown senior to Aamir Khan but in real life he is 23 years younger than him.
The pregnancy scene was actually planned for Munna Bhai M.B.B.S but Rajkumar Hirani changed his mind and years later used it in 3 Idiots.
Shahrukh Khan who chose My Name is Khan referred himself as the fourth idiot for not being a part of such a great movie.
Kareena Kapoor wanted to work with Rajkumar Hirani and she herself approached him for the role.
Boman Irani initially refused to play the role of VIRUS and even recommended Irffan Khan's name but later agreed to be a part of the film.
The song “Zoobi Doobi” is actually a tribute to Raj Kapoor and his hit song “Pyar Hua Ikraar Hua” from Shree 420.
According to Rajkumar Hirani the original cast of the film was Shahrukh Khan, Saif Ali Khan and John Abraham.
At one time there were reports that Hrithik Roashan, Abhishek Bachchan and Saif Ali Khan were considered for the film.
Aamir Khan and Javed Jaffrey did not share a single scene together in the film. Before this they were seen together in one film which released in 1990 and was titled, “Jawaani Zindabad”.
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