I had one last night.
We were in the forest setting up a stage as a school project, I just figured it was for theatre. I went to my car to get my wand for Harry Potter, because that's an important component in setting things up. I saw some guy in a Tigger costume and realized, "Oh, Mika's here!"
There were two different sides of the audience facing each other, like a living room without a TV. But the only people there for the concert were the people I went to school with. I found one of my best friends in the audience, smoking, which you'd never see her doing. She was hand in hand with someone who appeared to have no face.
I went "backstage" to meet Mika. He was in a really white room with a blue blanket draped over him and another faceless, buff man sitting next to him. He was crying into the man's shoulder and seemed kind of relieved when I got there. The only other thing that was in the room was a movie showing on an invisible projector. Mika motioned for me to sit in the grey plastic chair next to him. I did, and watched the really serious movie with him. All I remember about it were blood bags hanging from a fine metal wire. When the movie ended, I ran out the door after realizing I was late for class and sprinted through a school I didn't recognize, heading straight for the restroom.
Only then did I realize I'd been wearing a costume the whole time. When I got to the restroom, I found my clothes in a dirty pile on the floor, my bright red jeans ruined from people stepping on them, my underwear gone and my shirt was dangling from the mirror. I put these clothes on and ran into the hallway, then I woke up.
It was strange!