The curse of the Ninth Symphony is one of classical music's juiciest bits of lore, akin to rock music's "27 Club"—and like that legend, the truth of the matter is more complicated than the gossip suggests. Although composers like Mozart and Haydn had completed dozens of symphonies, the "curse of the Ninth" is said to have begun with the death of Beethoven, likely the most famous symphonist, who never completed his Tenth. Other big names— Dvořák, Vaughan Williams, Bruckner, Schubert—are often cited as victims of the curse, but the lore probably owes its enduring power to Gustav Mahler, who was allegedly so superstitious about the curse that he published Das Lied von der Erde as "A Symphony for Tenor, Alto (or Baritone) Voice and Orchestra" rather than giving it the number 9. The irony, of course, is that he died after completing his Ninth Symphony—which would have been his Tenth if he hadn't been afraid of the curse!
"It seems that the Ninth is a limit. He who wants to go beyond it must pass away. It seems as if something might be imparted to us in the Tenth which we ought not yet to know, for which we are not ready. Those who have written a Ninth stood too close to the hereafter." — Arnold Schoenberg