Ask most people what makes Allegri's Miserere unforgettable and they will point to one moment: a treble voice soaring to a piercing top C above the rest of the choir. That note, as familiar as it now is from recordings, is not in Allegri's own surviving notation. What the composer actually wrote down was a plainer, lower-lying setting; the elaborate, high-flying ornamentation that modern audiences associate with the piece grew up gradually as an oral, unwritten performance tradition inside the Sistine Chapel choir itself, singers adding their own embellishments to a piece that was, for most of its history, never fixed on paper for outsiders to check against a fully specified original.
That means the version fourteen-year-old Mozart transcribed from memory in 1770, and the version most modern recordings and performances present as "the" Miserere, are themselves closer to snapshots of an evolving oral tradition than to a single fixed authorial text, in a way not unlike how a folk song can drift across generations of singers even while everyone involved believes they are performing the same piece throughout. Musicologists reconstructing Allegri's presumed 1630s original from what documentary evidence does survive generally produce something notably plainer than the version audiences expect to hear today.
None of this has dented the piece's popularity. If anything, the gap between the historical Allegri and the performance tradition that grew up around his piece over three and a half centuries has become part of the story audiences are told when they encounter it, a rare case of a work's best-known feature being, by most musicological accounts, largely someone else's addition.