Saturday Things You Might Not Know

Excerpted from Wikipedia:

Miserere by Gregorio Allegri is a setting of Psalm 51 (50) composed during the reign of Pope Urban VIII, probably during the 1630s, for use in the Sistine Chapel during matins on Wednesday and Friday of Holy Week. It was the last of twelve falsobordone Miserere settings composed and chanted at the service since 1514 and the most popular: at some point, it became forbidden to transcribe the music and it was only allowed to be performed at those particular services, adding to the mystery surrounding it. Writing it down or performing it elsewhere was punishable by excommunication. The setting that escaped from the Vatican is actually a conflation of verses set by Gregorio Allegri around 1638 and Tommaso Bai (1650 - 1718, also spelled "Baj") in 1714.

The fourteen-year-old Mozart was visiting Rome when he first heard the piece during the Wednesday service. Later that day, he wrote it down entirely from memory, returning to the Chapel that Friday to make minor corrections. Some time during his travels, he met the British historian Dr Charles Burney, who obtained the piece from him and took it to London, where it was published in 1771. Once published, the ban was lifted and Allegri's Miserere has since become one of the most popular a cappella choral works now performed. The work was also transcribed by Felix Mendelssohn in 1831 and Franz Liszt, and various other 18th and 19th century sources survive.

Mozart was summoned to Rome by the Pope, only instead of excommunicating the boy the Pope showered praises on him for his feat of musical genius.