I took part in a master class given by classical guitarist Christopher Parkening many years ago. He stopped me after a few bars to tell me my guitar was out of tune. I had to tune in front of him and the rest of the class (no electronic tuners allowed). However, I was one of five students chosen to play at a post class mini-concert. And, I did use my electronic tuner (one of the first on the market back then) to tune back stage.