As I've mentioned, I show up for the regular Washington State Patrol's anti-riot/protest training sessions, to be one of the role-playing protesters. It's sort of fun getting water-cannoned, tear gassed, batoned, and tackled/arrested, all by people trying to learn to do so in a reasonably-safe fashion for all concerned.
In my chats with the WSP folks, they also believe there is a core of agitators who show up at pretty much every protest with the intent to incite disharmony of various sorts. They spend a fair bit of effort trying to identify these "frequent flyers"...