Can think of a few - like Pink Floyd doing a world premier of Atom Heart Mother in a field at dawn with a full orchestra and choir; Pink Floyd playing unplugged because the lorry with their gear had broken down; a young Jethro Tull as a supporting act (to The Searchers) and blowing the main band away); Jefferson Airplane waiting for a thunderstorm to end, but Jorma and Jack (their guitarist and bassist) doing a gig as Hot Tuna standing on rubber mats in the rain and lightning anyway; The Who in their final warm up gig before recording Live at Leeds; Leonard Cohen in his pomp; Fairport Convention with Sandy Denny and Ian Matthews.......then I think, yeh, done all that, I was there!
Would have liked to have seen The Beatles, the Pretty Things and The Small Faces though. Oh, and the original Velvet Underground.
I saw Hot Tuna this summer as part of a tour. Absolutely amazing. Last time I had seen them was 1992 in the local boozer at Uni in Colorado. Jack and Jorma can still do it, even today.