Not specifically instrument related but a band I play with occasionally were looking for the correct room in which to set up their PA. They opened a door which turned out to be at the back of a stage upon which were several people addressing some sort of seminar. A very nice chap asked the participants to hold on whilst he directed the band to the right room. Only then did they realise that the "nice chap" was the Archbishop of Canterbury.

On another occasion they were playing at a miners gala in Sheffield when a man from next door came in to complain about the volume of their drummer. He wouldn't be persuaded that they didn't have one until they showed him the stage. They all went outside and, sure enough, they could clearly hear drums - Bruce Springsteen was playing a football stadium in Sheffield five miles down the road.