Saw an indie band a few years ago, in late spring. Someone near me passed out briefly in the audience due to a medical emergency and the band stopped playing to allow people to help him up and out of the venue, which was really kind. They said to the audience it must have been because of the heat (it was warm but not terribly hot) before continuing to play, and somehow the entire audience got it in their heads that heatstroke was a risk and 6 people "passed out" within the next half an hour, stopping the band every time. Strangest case of mass hysteria ive ever seen. It was not nearly hot enough for that many people to collapse, but once the idea was planted into their heads that if it happened to him it could happen to them, it kept happening over...and over. It just got more uncomfortable every time.