As per Indian patent act, a patent cannot be granted without disposing pre grant oppositions, if any. There were some cases in indian law for such incidents, where patents had been granted without disposing pre grant opposition.
1) The valganciclovir case was the prominent case where controller had granted patent to Roche without giving opportunity for hearing to the opponent (Network of HIV+ people). This case could be read in detailed here. Madras High court set aside patent grant and asked controller to hear pre grant opposition and dispose with merit. (order)
2) The next case was a patent related to Humira granted to applicant Abbott without disposing pre grant opposition filed by Glenmark. In this case, IPO sent a letter to patentee as patent is cancelled. Abbott had filed writ to delhi high court and high court asked controller to consider the pre grant opposition as post grant, because pre grant opponent is eligible person interested to file post grant opposition. (SpicyIP) (Order)
Another related order in this line is Neon Laboratories Vs Troikaa Pharma. Toikaa Pharma is Ahemadabad base indian pharma company involved in specialty pharmaceuticals. Recently launched Diclofenac injection (Dynapar) and filed patent application
95 96/MUM/2005. Neon Lab has filed opposition on 96′ application and requested hearing. Controller without hearing Neon lab, has granted patent to Troikaa. In this order, Bombay high court set aside patent grant and asked controller to hear Neon lab.