@LonelyDadThe 'best' jam-session I remember was at a summer music festival. It toured the country over the course of about a month. two of the 'adjacent dates' were separated by a 12 plus hour drive with a significant flood in the middle so much of the 'fodder' acts gear couldn't make it within the 1 day time frame between the two 'shows'.
It was on a 'main' stage in the middle of the day and one of the acts, really one main guy with long term 'session' musicians in tow had no kit. So the lead guy beg/borrow/stole a semi-acoustic, what he is known for playing and riffed in situ for the whole hour he was scheduled for. the whole crowd ate it up. very impressive performance seeing he didn't play anything remotely recognisable as anything he was known for playing/recording. it was completely off the hook and cuff. very well done.