I'm going on whatever pops into my head...(that CAN be a bit risky, buuut....) _________
From what I understand, while they were filming 'HELP', Paul was playing the 'Scrambled Eggs' snippet that was running in his head on the nearby set piano over and over again, until their director, Richard Lester, finally had heard ENOUGH...
Mr. Lester said later that he told Paul, "If you play that bloody song ANY longer, I'll have the piano taken off stage! Either FINISH it or give it up!"
At least Paul can take comfort in the face that 'Yesterday' has been covered more than any other Beatles song...2,500 times!