Ben Byram-Wigfield and The Marian Consort demonstrate three versions of the Miserere by Gregorio Allegri (1582-1652). In this video, they tell how his own Sistine Choir version of harmonized plainchant became surrounded with myths, mistranscriptions, and performance mistakes that produced the legendary and beloved version we all know:
(Thanks to Peter Kwasniewski for bringing this to my attention.)