The Rolling Stones 1969 US tour was supposed to culminate with a free concert at Golden Gate Park in San Francisco, but the city council pulled out and, at the last moment, the concert was moved to the NoCal wasteland
Not a good idea, as it turns out. The organization was a disaster. Fights broke out in the crowd, but the security staff, comprised of Hell's Angels, proved even more problematic.
The Grateful Dead had the brilliant idea to bring in the Hell's Angels to help keep things cool, but the security force instead beat down the poor hippies and stomped on the whole peace love and happiness vibe. During Jefferson Airplane's set, singer Marty Balin was punched in the face by one of the Hell's Angels, and as The Stones played on while the crowd and the Angels clashed, a member of the audience was stabbed to death by a member of the 'security force'. Some people think this was the event that signified the end of the 60's, but since it was in 1969 anyway, that's not really much of a stretch.