One of Così fan tutte's zanier episodes occurs at the center of the first-act finale. Despina, disguised as a doctor, marches on stage, produces a magnet, and works a miraculous Mesmeric cure on the ailing Albanians. These antics obviously strain credibility even for a patently artificial work like Così fan tutte. So does her greeting: “Salvete, amabiles, / Bones puelles,” she announces, in mangled and untranslatable Latin (it runs, loosely, “hail, friends, good girls”). The solecism “bones puelles” does not appear in Da Ponte's libretto, which got its Latin right, as “bonae puellae.” A clear indication of Mozart's intervention in a Da Ponte text occurs infrequently, which is why these two words have attracted more critical attention than they might otherwise warrant. This grammatical alteration seems to represent an instance where the visions of the composer and the librettist part ways.

But are their conceptions really that distant? Arguably, Mozart's change only follows the lead of Da Ponte's text, which has low comedy written all over it. Immediately following this passage, the sisters observe that the weird dottore speaks in an alien tongue (“Parla un linguaggio / Che non sappiamo”), a remark indicating that Despina's salutation is meant to come off as gibberish whether her Latin parses or not. Even so, almost all interpretations of this passage posit a conceptual rift between Mozart and Da Ponte, and, unfailingly, they cite social verisimilitude as a guiding principle behind Mozart's alteration.