There Really Is "nothing new under the sun".

"Flash Mob" is a term that currently means a coordinated group surprise event or unexpected activity, in unison, in public.  Included are such things as a large group seemingly at random breaks out in the same song; a large seemingly unconnected group suddenly turning into "statues"; or the same tactic with dance or some other wacky idea.  Most of these are intended to be harmless but frequently are disruptive, annoying, or worse.  We think its a "hip" modern idea.  The first flash mob recorded in history is Hannibal's use of a "Freeze Mob" at the Battle of Cannae, grabbing the element of surprise when they "Un-Freeze".  He was able to pull it off several times before the Romans got wise.

Depiction of the Battle of Cannae - first recorded Flash Mob - the old Freeze/UnFreeze gag, deadly clever in this case.