According to Ritchie Blackmore, Graham Bonnet from Rainbow was told repeatedly not to cut his hair and to grow it out. Ritchie Blackmore says he'd gotten someone to guard Graham's hotel room so that he wouldn't sneak out to get a haircut. Graham Bonnet then proceeded to climb out of the window, as Blackmore tells. He found it hard to be on the same stage as that haircut and had to suppress the urge to smash him with his guitar.
According to Graham Bonnet, he simply got it cut as he passed by a barbershop. When he came on stage that evening with his new hairdo, Ritchie was shocked and played from behind the stack of amplifiers the entire show. The following day, Ritchie called for a band meeting and pointed out Graham's hair was too short and that he must've gotten it cut as a personal attack to Ritchie. Everyone laughed at him.
Bonnet also says they had rooms on the seventh floor of the hotel and claims that, everytime Ritchie tells the story, he adds something new.
Sources: Ritchie Blackmore's side and Graham Bonnet's side