Have only experienced this once. Bloke came up to me between sets and told me my technique was all wrong. He said I should be reaching over the neck and fretting with my thumb (!) so I politely pointed him to the guitarist, who according to him had his eq all wrong. We're pretty hot on not hogging each others frequency, so during the next set the guitarist pretended to change settings without altering anything and got a big thumbs up from the numpty. Was told after the gig by a couple of locals that the bloke was an expert on everything and a constant source of entertainment for them