I was in Covent Garden once when a man placed a ghetto blaster on a nearby table, turned it on and started to sing. A woman appeared above, at the railings, and they performed a duet. It was wonderful and felt like an impromptu performance.
This is even better:
Last November, at the central market in Valencia, opera singers disguised as shopkeepers were selling produce at the various stalls there. Verdi's Il Travatore starts playing over the loudspeakers & they burst into song. None of the shoppers has a clue what's going on.