Many years ago, I was standing on the wings of a stage while the band Social Distortion played a set at a festival show. (My friend was an audio guy) They broke into "Ring of Fire" and out of the corner of my eye, I noticed a fairly tall, but frail older gentlemen dressed all in black standing next to me. I saw him smile as he took notice of me singing along and enjoying the song. At the end of the next song (Story of My Life), Mike Ness, the singer for Social Distortion walked off my side of the stage and did a double take as this older gentleman shook Mike's hand and said "You did her proud son"
Seems I had been standing next to Johnny Cash for almost 20 minutes and didn't even realize it.
Johnny Cash was in town for another show, and somehow he found out Social Distortion was playing that afternoon. He had heard their cover of "Ring of Fire" and decided to hear it live.