I'm not going to count things like clothing optional (or marginally-optional) swimming areas, or parties related to That Festival In The Desert, or things like the Love Parade where people are flashing everybody, or the couple who run around naked at Grateful Dead shows.
But there was one concert I was at in the late 90s where I was dancing near a girl wearing a yellow tube top who was dancing enthusiastically. At one point one of her boobs popped out, and she pretended not to notice, and I smiled at her, and eventually her other boob popped out and she pretended to have suddenly noticed and covered up. This went on for a while, with both of us having a good time with it. After it got dark she took off the shirt entirely.
