This had happened some 25 years ago when Shah Rukh Khan and Kajol were shooting for their first ever film together "Baazigar".
By 1993, SRK had already done few films and he was a well knows star after the successs of Diwana. He was also a familiar face for Doordarshan viewers. But Kajol was new and this was her first ever film.
The acting skills, or rather lack of it, of Kajol didn't impress SRK any bit and he immediately told her that she should learn some acting. He also called her idiot!
Kajol was not amused and asked SRK to mind his own business, though later she realized that SRK had a point and then she worked hard on her acting.
The actress recalled this incident in an interview given to a film magazine recently.