Don't know if this counts as a concert, but I was cutting through the Alto Palermo Shopping Mall to avoid a recent heavy rain when, without warning, a THX Sound System started blasting orchestral music and one the mezzo sopranos of el Teatro Colón showed up out of nowhere and began singing the famous solo from Carmen as she snaked down from the third floor stairs to the main floor in an expensive mink coat. This segued into sort of a half hour of a local baritone, tenor, and another soprano performing the show stoppers from Turandot, The Magic Flute, and the Barber of Seville. Then the sort of impromptu performance was over just as quickly as it started with the singers disappearing into the crowds of shoppers.
It was sort of like those pseudo spontaneous group dances that occur in public spaces without warning.