Saw it as well, I was on the last row of first deck and the second deck was bouncing a solid 6 inches at the end of the overhang. Everyone around us saw it and couldn't believe it. Not a sway, but a bounce. It was pretty wild. Only lasted about 5-6 oscillations. No, the observers are not drunk, and I've never noticed that happening).
Edit: To confirm others posts -- it was at the very end of the war hymn as the band played at a very quick tempo. I'm an ME (not CE), but I was amazed concrete could flex that much.
[This message has been edited by tamuaggie11 (edited 9/14/2013 2:11a).]
[This message has been edited by tamuaggie11 (edited 9/14/2013 2:21a).]