Flash mobs initially involved dancing in the streets, freezing like statues or even giant pillow fights.
But then something else happened. Bicyclists in San Francisco and Los Angeles started blocking traffic to protest lack of respect from motorists. A spontaneous snowball fight during the winter's epic snowstorm in Washington, D.C., went sour when a plainclothes policeman pulled out a gun.
And, then, two weeks ago in Philadelphia, touristy South Street was the scene of a flash mob gone bad. Foot traffic on the street changed in minutes from normal Saturday date night to an onslaught of mostly young people seemingly intent on destruction.
Tom Vasiliades, owner of South Street Souvlaki, caught some of the action, both inside and outside the restaurant, on surveillance video.
Startled customers huddled inside. Someone locked the doors.
"The people, we locked the doors and they stayed here until the streets were under control," Vasiliades said. "Everybody was afraid. ... Really afraid, yes. Customers and everyone."
Noel Meneses, who manages a pizza parlor down the street, said the crowd was out of control.
"What I see was like young kids getting on top of the cars that were parked on the side of South Street," Meneses said. "They were jumping on top of the cars, smashing windshields, breaking windows. ... My customers got so scared, so what I had to do is lock all the windows and the doors and not let anyone in or out for at least half-an-hour. They were so scared and these kids started to come in. ... I've never saw that before. First time in my life that I see something like that."
Police had received an anonymous tip from a parent, so they were ready. They quickly dispersed the crowd but the incident frightened customers and business owners.
"On weekends, we close at 11 [o'clock], and I had to close at like 10 because the police took everybody out of the street and kicked them out," Meneses said. "There was nobody on South Street after 10. ... They had moved all the kids."
Having spent the past two years fighting to free Philadelphia from its nickname, "Killadelphia," and lower the crime rate, city officials, including Police Commissioner Charles Ramsey, are taking a hard line against violent flash mobs.
"Judge [Kevin M.] Dougherty, who sits on Family Court [Juvenile Division] here, took a very strong position," said Ramsey. "He's my new hero, because he really took a very tough stand on these kids. We don't want to give them records, we don't want to, or do anything that will harm them in the long run, but at the same time you can't just assault people.
"You can't destroy property of others. Come down and enjoy yourself, but you can do it without causing a lot of damage and a lot of ... issues for business and others."
It wasn't Philadelphia's first brush with the ugly side of flash mobs.