Granted it sounds apocryphal but I've always liked the story of the Rolling Stones party with Jagger screaming (in Stella Street voice) for "my bladdy drummer" and said drummer turning up and decking him. I'd like to believe that happened.

Incredible String Band, Pink Floyd, Soft Machine.
Also, Grateful Dead at an Acid Test.
Sandy Denny fronting original Fairport Convention line-up, anywhere,
Jimi Hendrix Experience - Monterey.

The birth of British rock and roll; the dawn of almost everything we discuss here.

when Robert Johnson had his appointment with you-know-who (no, I don't mean Voldemort!)...

...the on-stage explosion of Mick Shrimpton.

From the Romilar-infused quill of Lester Bangs:

Lou took Metal Machine Music straight to the top, to Kenneth Glancy, president of RCA Records, and worked his way down from there. Office to office, and every one he goes into he just presses the button and out comes ZZZZZZZRRRRRRRREEEEEEEEEGGGGGGGGGRRRRRAAAAARRRRRRRRGGGGGGGGG
HHHHHNNNNNNNNNNNNIIIIIIIIIIEEEEEEEERRRRRRRRRRRRR... all the way down the line ebrey one-a-dem egg-zecks past de bucks. "Sure, anything, just get it outta my office!"

musical heroes, but would have enjoyed being present (at the Santa Barbara Bowl ?) for this:

and a Genesis gig pre-Duke (maybe with Gabriel, or the Paris shows on which Seconds Out draws); and also Leonard Bernstein's last appearance at the Proms in the late 80s iirc

http://en.wikipedia.org/wiki/Voodoo_Chile

I'd love to have seen Elvis do the up-tempo first take of 'Blue Moon Of Kentucky' at the Sun Records studio.

Or sat in on the Miles Davis 'Kind of Blue' sessions.

Or witnessed Jon Anderson outlining 'Tales From Topographic Oceans' to his Yes bandmates might be an interesting one - he apparently turned up with most of it in his head and proceeded to outline the main themes and pieces on the piano. I bet Rick Wakeman was impressed.

Or looking in on David Bowie persuading Mick Ronson and the other lads from Hull that putting on make up, tight satin pants and glitter would aid the musical credibility of The Spiders From Mars.

Or listening to Crosby, Stills and Nash sing harmony together for the first time - just to see the expression on their faces.

In Berlin to witness the recording of Low / Heroes.

Or sitting in the corner of the room while Fairport rehearsed Liege & Lief.

In the audience at the filming of The Last Waltz.

To be at Mama Cass's house in Laurel Canyon (if that's the right location - memories are a little hazy) when CSN first discovered what they could do with those three voices and a guitar.

Or with Adman at The Last Waltz.

But if I could only have one it would be to be standing on the rooftop of 3 Saville Row on 30th January 1969.

from Chas Chandler at the Cafe Wha listening to a guy called Jimmy James.

Duke Ellington and his Orchestra appearing, but they were very much last year's (or last decade's) thing and proceeded to play a storming set that blew the place apart.

Always found this part of the film fascinating , especially the gent with the pipe wandering over the rooftops vying for a better view.

to have seen XTC live. Had a ticket once but the show was cancelled. Rest history etc.

I think being around to watch the ragtime to jazz
process described by Goodalll about 1 minute onwards here would be wonderful