John Sebastian attended Woodstock as a spectator, but after a downpour delayed Santana's set, they asked John to grab an acoustic guitar and play a set while the stage dried. The crowd knew him as the lead singer of the Lovin Spoonful, but Sebastian played songs he hadn't released yet - along with a few Spoonful classics.
There's a rumor that he was trippin balls when he performed because he was there as a spectator and wasn't expecting to have to play.
View: https://www.youtube.com/watch?v=rBXL7FaPod4&ab_channel=MyInnerEyeMike