Karan Johar is indeed one of the most acclaimed and established young producer of Indian cinema and rarely a Bollywood actor would refuse his movie. But this has happened with Saif Ali Khan as he denied the film produced by director Karan Johar's Dharma Productions in which he was being given a lead role. The Kal Ho Naa Ho star excused himself from the film, due to unavailability of dates.

The film not yet named is the directorial debut of Karan's assistant Tarun Mansukhani, who has assisted him since Kuch Kuch Hota Hai.
Busy Saif is currently engaged in Abbas Mastan's thriller Race. After that he has committed shooting for Yashraj Films' Tashan and that will be followed by Kunal Kohli's next movie with Rani Mukherjee in September-October.
As a matter of fact Mansukhani shoot was coinciding with Saif's October shoot as Saif has already given his commitment to Kohli for that month.
Earlier Saif, Abhishek Bachchan and Aishwarya Rai were finalized to play main roles in the film. But now, Saif has opted out because he doesn't have the dates to spare, therefore now Dharma Productions is left only with Abhishek Bachchan, as he has given his approval for doing the film. As far as Aishwarya Bachchan is concerned she too has not yet confirmed her role.
Karan Johar has understood Saif's tight situation and therefore he also did not persuade him for the movie. Now the young producer is planning another film with Saif early next year.
Source: Apunkachoice