This is one of the things I love about festivals
The best moments often take you completely by surprise. I've seen a few impromptu sets by bands who have either already played or are due on later.

Best ever was Billy Bragg and Bill Bailey strumming out a brief acoustic set round the back of the Leftfield stage at Glastonbury a couple of years ago while they waited to go on, while me and about ten other people sat watching, drinking cider and providing harmonies
( chart cat has an itch, Tue 9 Jun 2009, 6:13, Reply)