A flash mob is a group of people who gather in a public place, perform a strange and/or pointless act and then disband. Former acts have included large choreographed dances and a mass scale pillow fight. Flash mobbing was created by then Harper's Magazine senior editor Bill Wasik as a way to make fun of hipsters. Ironically, the activity became a huge success amongst the very people it was aimed to mock.
Famous Brisbane flash mob moments include World Freeze Day, when more than 200 people imitated statues in the middle of Queen Street Mall, the zombie march of 2008, which set an unofficial world record (1500 people dressed and acting like zombies) and a group dance performed to a Cold Chisel mash-up that opened the Brisbane Writers Festival in 2009.
There was a need for secrecy in the early days of flash mobbing, as the targeted venues would sometimes hear about the potential act and would take security measures to thwart the mobs efforts. Participants found out through internet postings and text messaging. To stay informed, check out Brisbane Flash Mob, Flash Mob Brisbane Elite (you won't be allowed to view the content, but you can make an enquiry about the group) and this website, which provides a text line to which you can subscribe.
Nowadays, though, people are generally more good natured about flash mobs and the organisers can advertise more publicly. Keep an eye out in Brisbane street publications like Rave and Time Off for information about upcoming flash mob events.