The final movement of Beethoven's Ninth Symphony. My mother would have loved this - she played the bassoon in her high school and college orchestras, and as you can see, the whole thing starts out with one. This is apparently one of the hotter videos on YouTube at the moment, and y'all can certainly see why.
Is it an authentic flash mob? Apparently not:
Commenters on the video have rightly noted that it does not appear to be a flashmob in the purist sense of the word. Watching the video a few times over, it does seem that the audience was aware something might be happening that day -- although their surprise and delight does seem genuine, so presumably they were not told exactly what to expect. Also, it is unlikely that the audio track that accompanies the video was recorded at the same time as the video because the sound is so good and the players do not appear to be miked.
And then there is the inescapable fact that this flashmob was staged by a Spanish banking consortium called Banco Sabadell in honor of its 130th anniversary, so it is not exactly a coincidence that the orchestra chose to gather in front of the Banc de Sabadell.Does it matter to you? Does not to me! Sometimes we find silver linings in the cloud of our brave new world, and this sort of thing is one of them. Imagine trying to explain a flash mob doing this, the explain the institution that brought this video clip to us, over the internet to, oh, my parents, much less my grandparents.
Special thanks to jed, who mentioned this in his comments to the post about Journey of Faith's flash mob performance.