Happened to me about ten years ago. I had been playing with The Wirebirds for a year and a bit, getting plenty of gigs. My mother -in-law was living with us back then (no longer around) and had a carer come in a couple of times a week to take her to the shops, etc.
One day, there was a new carer putting MiL's shopping into the fridge, and MiL mentioned that she (the carer) liked singing, so I naturally mentioned the band and she asked if she could come and sing with us sometime. Then she mentioned her previous band and the tours, hanging out with the guys from Led Zeppelin, etc, and I was sure she was a dreamer.
Anyway, she came along to our rehearsal a week later, but then I had found out who she was and told the band, they basically said it was just a fantasy on her part. Then she came in to the rehearsal room, tiny, shy, no way a rock star. She asked if we knew 'Rock Me Baby' (well, we wouldn't have been much of a blues band if we didn't!), she asked if we could do in A, which was our usual key for that song. We played the intro, and I could see the rest of the band waiting for her squeak out a few words, then she grabbed the mic and our jaws dropped. She could sing the blues like Janis Joplin. We were getting better gigs on the back of her being with us, people were bringing her old records to our gigs to get them autographed.
She was Jenny Hahn, front woman for Babe Ruth, and we played dozens of pub gigs and local festivals with her.
Her she is singing Janis's 'Move Over'. I'll never equal the thrill of standing a few feet from that voice!