Many years ago, I saw The Beach Boys play at Knebworth. Who's the "weird" one? Brian? Well, he was acting strangely during the gig. Wandering around, on and off stage. Not really doing much. Anyway, to this day I swear he was replaced with a doppelganger half way through.
I may have been under the influence of certain substances at the time. But, I'm almost certain that this happened.