I saw Peter Yarrow (of Peter, Paul and Mary) at a folk festival back in 2014. The local high school chorus was there to sing backup on "Light One Candle." They'd rehearsed the song, knew the words, and were ready to go...standing behind a curtain so he could introduce them when he started the song.

Right before the curtain went up, he said "but what they don't know, is that they're going to come down here and join us all to sing Puff." As in Puff the Magic Dragon, perhaps his best known song. Before the show, I was talking to the kids backstage, and they really didn't know who he was. So I said "you know the song Puff the Magic Dragon?" They all nodded. "That's the guy."

So the curtain goes up, they sing Light One Candle, and the kids gather around Pete to sing Puff. Just after the first chorus, he stops and points to a student at the back...

"She's reading the lyrics from her iPad!" Because she was.

He summoned her down to the mic and said "we're going to sing the next verse together." So this girl (with her iPad) got to sing a verse of one of the most famous songs in the world with the original artist. She did great.