Saw him loads of times between 1977 and 1992 (approx.).
9 Dec 1984, a Scottish promoter had organised a Concert for Ethiopia in Edinburgh. The bill was, in order of appearance;
A local artist who I don't recall, then;
Set 1 John Martyn
Set 2 Jim Diamond, w Clem Clempson - Guitar, Simon Kirke - Drums, Ronnie Leahy - Keyboards, Chrissie Stewart - Bass
Set 3 Rick Wakeman, w Billy Bremner - Guitar, Tony Fernandez - Drums, Chas Cronk - Bass
Set 4 Bert Jansch, w Richard Newman - Guitar, Lauren Albach - Vocals
Set 5 All-star band with Jack Bruce and Rory Gallagher w Ian Stewart - Keyboards, Charlie Watts - Drums, Dick Heckstall-Smith - Saxophone, Willie Garnett - Saxophone, Don Weller - Saxophone, John Pickard - Trombone?
Martyn's set went off OK, but during the all-star set 5, he barged onto the stage, made what he evidently thought were guitar-hero noises, strutted around in what he thought were guitar hero poses, and generally got in the way of everyone else. There's a few photos from the gig online, and in one of them he seems to be squaring up to Gallagher, as if to challenge him to a shoot-out. Some accounts relate that he'd been in a pub next door for the duration of sets 2 to 4, and got hammered.
At one point, I could see him and a bunch of roadies offstage to the side of the PA stacks. He was walking around with his guitar on, and seemed to be exchanging angry words with the roadies. One roadie was casually sitting on a flight case, and Martyn took off his guitar, grabbed it by the neck, and was in the process of swinging it at the roadie when the rest of the crew jumped on him and bundled him away...