The ending
The orginal producer of the play, Herbert Beerbohm Tree, introduced the "happy ending" to the dismay of Shaw. "My ending makes money; you ought to be grateful," scrawled Tree. "Your ending is damnable; you ought to be shot," snarled Shaw. In 1938 producer Gabriel Pascal sneaked in the same happy ending over Shaw's objections. And of course the musical version "My Fair Lady" did the same.
Shaw became the only person to win an Academy Award for best screenplay and the Nobel Prize in Literature. My guess is he owed the awards to that happy ending he hated!