Flash mobs can be innocent enough. Sometimes the flash mobs are goofy—hundreds of people meeting in a bus, rail or subway station and then dancing together as a group. Sometimes they are sublime—like when the Copenhagen Philharmonic members met on a train and performed Peer Gynt for surprised passengers.
More often they are criminal. Groups of young teenagers have been planning and executing bold robberies—“flash robs”! The teens use social media outlets—mostly Twitter—to organize themselves.