Kids who can't handle their drugs
If you're going to do drugs, learn how to do drugs. "Doing drugs responsibly" may sound senseless, but believe it or not, there is a way to do drugs and not act like an asshole, and some concert-goers have been doing it for years. Decades, even. But every year, at every festival, somebody ends up being that guy. Some of the time it's hilarious (see below), but most of the time it's just annoying. Especially if you're within 50 feet of the person and you know they're not going to stop doing that little twisty worm thing any time soon.