Marc Shipper wrote a hilarious parody/spoof of the career of the Beatles, called "Paperback Writer".
In it is an account given by a fictitious musician who was hired by Paul McCartney for Wings. I'm quoting from memory here:
"So, Paul plays me a tune he's thought up. To call it "weak" would be charitable. He asked me to hang a riff on it. I could no more hang a riff on that tune than I could hang an elephant on a clothesline. So, I told him as politely as I could that I thought the tune needed some work."
"Paul kind of chuckled, and said 'So, you think it's Bull$%^ ?" And I thought, "Wow, this guy's all right and down to earth!" So, I said "Well, it's kinda Bull$%^..." And then he got really angry, and started shouting 'Who are YOU to tell ME that my stuff is Bull$%^#!!!! You're fired!!'
I imagine, in reality, that it would be very hard to diplomatically tell someone with a career like Sir Paul's where he is mistaken.
Oh, as a side note, that same book mentions an album that was made by Pete Best and Jimmy Nichol. It was called "The Beatles Told Us to Beat It!" An entire album of only the drum tracks from Beatles songs, it was an avant garde classic at the time, and is now a highly sought after collector's item.