The only accident I've witnessed, which didn't turn out as bad as it looked, was at the 1968 Jazz & Blues Festival at Kempton Park. Saturday night, we'd got seats, but lots of people had climbed on the roof of a stand at the rear of the ground. It collapsed, although nobody was aware of what was happening at the time, until the blue lights started appearing. John Gee was on the stage, trying to get through to the audience. We just shouted and cheered louder - Arthur Brown was due on and we thought it was one of his spectacular appearances (the year before he'd been lowered in on a crane - anybody remember that).
One of the things I do remember about the 60's was the unbelievably dangerous things with did with electrical equipment. Nobody was suppposed to get killed, but they sometimes did. Accidents short of death were commonplace. A crack off an unearthed mic full of gob was potentially lethal, but hardly rare. Those were the days.