Inside a Chuck E. Cheese, a little kid in front of a "Toy Story"-themed birthday cake gives a shy smile as his friends and family gather around, getting ready to sing "Happy Birthday."
It seems like a sweet scene until BAM! The fists start flying. Chaos ensues and the tears start flowing.
At these family restaurants, it is not uncommon for adults to start fist-fighting during kids' birthday parties.
Just last month, a mass brawl broke out during a kid's birthday party at a Chuck E. Cheese in Commack, N.Y. Cell phone video of the fight captured one woman holding a baby in one arm and swinging at another adult with the other, while a little girl tried to break them up.
And these fights between grown-ups are not infrequent. Type "Chuck E. Cheese" into YouTube, and several videos crop up of uploaded cell phone footage showing birthday parties going very sour.
Barbie Clifton, a mother of five, said she was attacked at an Ohio Chuck E. Cheese during her daughter's birthday party.
"All I remember was hitting the floor, and my hair just being ripped and ripped and ripped, and just being kicked in the head," Clifton said. "The lady seemed to be infuriated over my children being on the picture machine so long."
At one Chuck E. Cheese location in Susquehanna, Pa., the local police said they were frequent visitors. According to Police Chief Robert Martin, local authorities were called to that location 17 times over the course of 18 months, starting in 2009.
"It got crowded, and kids would run into each other or fight over games, which would lead to parents getting involved in fights," Martin said.
At another Chuck E. Cheese in Brookfield, Wis., two women were arrested following a mass brawl involving 20 people on July 27. The brawl was sparked, according to police, by "one child taking too long to choose a toy."
Then, just a week later, at the same restaurant, a father and daughter were arrested after a fight broke out during a family birthday party. Witnesses told police the man hit his 20-year-old daughter "on the side of the face three times," according to authorities.
So why is this happening at Chuck E. Cheese? Two psychologists, David Schwartz, a professor at the University of Southern California, and Howard Stevenson, an associate professor at the University of Pennsylvania, viewed some of the fights on YouTube for "Nightline."
Schwartz didn't find the patrons' violent behavior surprising, saying, "birthday parties are really emotional situations. There's frustration and provocation."
Stevenson said people can bring unseen stresses to a child's party, especially if parents with a limited income have spent money to make their little one's day special.
"If you are overloaded with stress from life in general, then you might be less patient," he said. "And if you have very few resources, and this is your one opportunity to publicly state or show how good a parent you are ... then it's very stressful."
And that stress, Stevenson said, can quickly turn to violence over "the fact that your family is judging you on how well you do this, the fact that your children are looking up at you for a good time, the fact that they've been talking about it."
Added to the stress, even the happiest occasions can turn sour when it gets too noisy and too crowded. Throw in some alcohol to loosen tongues and quicken tempers -- Chuck E. Cheese serves alcohol at some of its locations -- and there could be trouble.
Problems can arise, "when you have family members coming for a birthday party, and perhaps it was a divorce situation or a separation, where there were already uneasy feelings between family members," said Susquehanna Police Chief Robert Martin.
Chuck E. Cheese declined to be interviewed but issued a statement to ABC News regarding this story. Read the full Chuck E. Cheese statement.
"While even one incident is too many, over 99.99 percent of approximately 65,000,000 annual guest visits at Chuck E. Cheese's go without incident," the statement said.
And regarding the recent incidents at the Commack and Brookfield locations, Chuck E. Cheese said that it had "reduced showroom seating and party parameters in response to recent incidents."
In addition, Chuck E. Cheese management said it had spent "$15 million to date on efforts to stop incidents like those.
"We are implementing an updated guest code of conduct, as well as rigorous market tests for additional security measures," the statement said.
But Chuck E. Cheese could perhaps stop serving alcohol.
There have been numerous other instances in which a celebration has quickly turned violent: sporting events, political debates, even Black Friday sales. In 2010, riots broke out in Los Angeles after the Lakers won the championships. In 2011, a peaceful demonstration in London against a police shooting ended in citywide mayhem that lasted for days.
"There's sort of a crowd mentality and a diffusion of individual responsibility," Schwartz said.
It seems so much more shocking when it happens at a Chuck E. Cheese, but we humans are wired to react to our environment, and a kid's birthday party -- the noise, the crowd, the stress -- has all the makings of a powder keg, waiting to explode. Full Statement from Chuck E. Cheese to "Nightline":
Statement in response to Nightline addressing Brookfield, Wis., and Commack, New York, incidents
While even one incident is too many, over 99.99 percent of approximately 65,000,000 annual guest visits at Chuck E. Cheese's go without incident. We have always taken altercations in our stores very seriously and have spent more than $15 million to date on efforts to stop incidents like these from happening. We are currently evaluating our most aggressive approach yet. We are implementing an updated guest code of conduct as well as rigorous market tests for additional security measures. Until recently, the Brookfield, Wis., location was incident-free in 2010, 2011, 2012 and half of 2013. What is more, there have only been two incidents at the Commack, New York, location in the last 10 years. At both locations, we have reduced showroom seating and party parameters in response to the recent incidents. As with all Chuck E. Cheese's locations, Brookfield and Commack are equipped with a state-of-the-art security camera system in hopes of deterring future incidents. We also conduct annual conflict resolution training with employees. Maintaining a wholesome, safe experience that sets a standard across our more than 560 locations is of utmost importance to Chuck E. Cheese's, and we will never stop working to improve how we operate our business.