Pete Seeger came over to Denmark for the 1990 (1991?) Tonder Festival. That was the only time the festival didn't end with 'Will the Circle Be Unbroken', for the crowd (2000+) started singing 'We Shall Overcome' and everybody came back on stage to finish it.
However, the really magic moment for me was the afternoon before, when the stage was cleared after a great concert, and here, too, the crowd started singing. Nobody came back on stage, so we sang four verses through on our own, as a homage to Pete.