I have seen this, or a variation of this, in several venues. One or two overseas; not sure if I've seen this particular video before or not.
On Saturday, April 24th, over 30 members of the Opera Company of Philadelphia Chorus and principal cast members from their upcoming production of La Traviata converged on the Reading Terminal Market Italian Festival in Philly.
Wearing street clothes and blending in with the crowd, the artists swung into action after the first orchestral strains of the famed "Brindisi" were piped through the market, giving a rousing, surprise performance for hundreds of delighted onlookers who were there to enjoy the Italian delicacies that the Market has to offer.
The four-minute piece won a thunderous ovation that included both laughter and tears from the audience. You can see it and hear it at: