I played a gig in a Glasgow bar a couple of years ago. The first set went down well, but one odd looking guy aged in his 60s stands at the side of the band facing the audience while nodding along to the tunes.
He then disappeared.
Second set starts, and about 10 mins in to it, the nodding sidestage man appears in the same place, only now he's holding a black padded holdall. He then proceeds to unzip it mid song, and produces a saxophone!! Which he then parps along to the tune we're belting out. At the end of the song he shouts,"YES, YES, YES!!!!", packs up his horn and vanishes.
At the end of the night we ask if anyone knows will the random player was - no one had the faintest idea who he was!!
Another oddity was being roped in to dep on bass for a "Christian rock band" (yeah, I know!!) on a gig in Aberdeen. Hours of driving to the gig, to be faced with a crowd in a church hall consisting of sheltered teens and early twenties types who look like they've never been allowed out in public on their own before.
Lots of shuffling from them during the band's set. About 20 mins in, they form a large circle facing each other, ignoring the band. Then, one by one they start doing forward somersault rolls from one side of the circle, across the floor, to the other side. And so it continues. Oddest thing I've ever had to play through.