Not a gig, but related.
I had a solo open mic spot about 8 years ago. I felt I hadn't practised the vocals at full volume enough (I can't do it much at home) so decided I would stop off in the car park of a disused sports centre in Luton and have a run through in the car.
So I parked up, got the acoustic out of the boot and got in the back seat.
I played though the songs a few times, when I became aware that there was now a police car parked on the other side of the car park… and that they seemed to be observing me.
I carried on, thinking best to act innocent. The male officer started to walk over . I carried on playing.
He had a real serious look on his face until he got a few feet from the car, then he started kind-of laughing.
He was laughing because he could see the guitar. It transpired that up to now they could only see the movement of my strumming arm, and thought I was entertaining myself in quite a different way.
I'm assuming they had some kind of tip-off as it would be too much co-incidence otherwise.
Anyway I did the open mic… went pretty well and I had a good "Something funny happened on my way here" anecdote to tell.