Lulu (opera)
Berg started to write the opera in 1929, but then he stopped working on it to write his Violin Concerto.  When Berg died, he had not finished the last act of the opera, he had only sketched it.  Schoenberg was asked whether he would finish the opera, but he thought it would be too much work.  When the opera was first performed in 1937 there were only two acts: the opera was unfinished.  Berg’s widow, Helene, said that nobody was allowed to finish it and that it should stay unfinished.  For more than 40 years, that is how the opera was performed.