Similar in Spocompton, WA long ago. Went to a Bryan Adams concert. Steel Breeze and some other band opened, Adams closed. He was in the middle of his last song, Everything I Do. He was solo on an acoustic guitar on a stool, curtain closed on the band behind him. A guy sneaks out of the curtain and holds his fingers to his lips in a shush and starts singing with Adams while walking up behind him. The audience is going crazy and Adams is looking around like WTF. He finally realizes Steven Tyler is right behind him and they stop and laugh, talk for a bit. Aerosmith was passing through on their way to Seattle and decided to crash the party. The curtain opened back up and we were treated to about 2 more hours of killer music with pretty much all the bands involved. No play list, just a jam. "Hey, you guys know this one? It goes like this..." They played all sorts of music from old blues to country to current hits by others.I saw him in 1984 with Heuy Lewis and the News on a Sunday night. They had a 45 minute jam session after everyone thought the concert was over.
My dates parents didn't believe us and I was not allowed to see her for about a week until her mom called me and apologized. One of her coworkers kids had been at the same concert.