These gatherings usually start out as manageable, well-behaved, fun events, celebrations, or parties. They can be viewed as rites of passage or just people blowing off a little harmless steam.

The trouble usually creeps in when there are too many people, alcohol arrives, and crashers make their presence felt. When word spreads through Twitter about THE party going on somewhere, things start spiraling out-of-control.

It may start with sports fans mourning their team's loss or celebrating a win. It may be an end-of-year school party, keg party, or impromptu street festival.

They may start out innocently enough, as the flash-mob phenomenon did a few years ago. Nowadays, Chicago and Philadelphia are on full alert because flash mobs in those cities have turned into violent, criminal mobs — running, rampaging, stealing, robbing and terrorizing the downtowns of both cities. In response, police in those cities have substantially beefed up their presence to dissuade and control violent flash mobs from occurring.

Flashmobs first gained popularity with their synchronized dancing, mega-pillow fights, and the like. It now appears that the thugs have interjected their own version of flash mobs: large roving bands of predators hunting their next prey.

Sideshows are another similar, street party turned violent that began in Oakland, Calif., and has now spread through much of California and elsewhere. Sideshows are a variation of illegal street races held in many cities throughout America.

Virtually every year, one or more Oakland sideshow ends in gunfire, near-rioting, fatal crashes, and assaults on police.

Both flashmobs and sideshows attract hordes of onlookers, curiosity seekers and participants. Responding police are almost always greatly outnumbered by these mobs, who gather strength from their numbers and defiance of authority.

Such legal and illegal gatherings have always been a headache for police. Many of them became annual events that attract people from throughout their respective regions. In the past, when the police showed up, they would leave quickly without a confrontation. They would then show up the following week at the same time and place. Good clean fun.

In the 1980s, there were hints of things to come with the advent of impromptu public crowd gatherings called "wildings." They involved the "word" going out to meet up at a popular public destination on a certain day and at a certain time and place. Major shopping malls were frequent targets in many cities.

One major northeast Ohio shopping mall experienced such gatherings every Saturday. The mobs quickly turned into dangeroud rampages ehere shoppers were knocked down, robbed and assaulted. The mob's members ran from one end of the mall to the other and then escaped through the exits.

Local police beefed up their numbers, but often needed to call for mutual aid from surrounding cities. The mall quickly gained a reputation as a place to avoid. Over time, enough shoppers avoided this mall — it had opened as the largest enclosed mall in America — that it eventually bellied up.

This sounds strikingly similar to the rampaging flashmobs in Chicago and Philadelphia. Involving large mobs of kids running and rampaging through major downtown department stories — stealing and terrorizing their way to their kind of "fun."

Meanwhile, the Oakland sideshows are such a menace that the Oakland Police Department , California Highway Patrol , and other law enforcement agencies routinely establish large contingencies of officers scrambling from sideshow to sideshow, making arrests, towing vehicles and in some cases quelling near-street riots.

Hopefully, flash mobs and sideshows are only short-lived fads that ultimately fall by the wayside. However, Oakland sideshows have been around for more than 10 years and show no sign of slowing down. If anything, they're getting increasingly violent and defiant toward police.

Meanwhile, on many American college campuses, end-of-school-year celebrations increasingly turn rowdy and violent. Street takeovers, bonfires, open (underage) keg drinking, and fighting culminated in rock and bottle throwing at police, who respond with pepper spray and arrests.

Let's not forget about the recent rioting in downtown Vancouver , Canada, after the NHL's Canucks lost the Stanley Cup Finals. I suspect most of the rioters weren't the least bit interested in hockey. Instead, this was their opportunity to take over the streets in open defiance of authority.

It took hours for the police to bring things under control. However, the damage was already done — cars (including police cruisers) were overturned and torched, store windows were smashed and looted, multiple arrests were made, and rocks and bottles were hurled at police.

Law enforcement agencies must remain vigilant, because small disturbances in the past have been known to explode into major rioting, as was seen during the 1992 Los Angeles riots , which was the deadliest riot in modern U.S. history.

In an era of cutbacks and downsizing in law enforcement, we must ask ourselves: will we be ready?

Related:

Flash Mobs Organize Robberies Via Social Media (video)

Stanley Cup Riot in Vancouver (video)