We may roll our eyes and shake our heads when opposing fans throw barbs (or rocks) at one another. It's just a game, we tell ourselves. It's really not that serious.
But honestly, who hasn't felt that powerful sense of pride when his college team wins, when his country earns a gold, when his hometown team brings back the big trophy?
And who, after his rival team lost a big game, hasn't Googled that rival's hometown newspaper to revel in the misery of opposing fans?
No, readers, for many it's not just a game.
And like any war, crossing enemy lines is risky.
Heading to the enemy's mother ship (read: stadium) without attempting to blend in (read: strip yourself of your team's jersey and cap), well, can be lethal.
Click on to see 10 stadiums which you should never wear an away jersey to.
Even New York Jets head coach Rex Ryan warned people not to wear away jerseys at MetLife.
Regarding an impending 2011 Jets vs. Cowboys game he said, "I don't know why they'd be here. They're coming into our stadium, it's probably not recommended that you wear Cowboys stuff, I would think."
Later, he retracted those comments, saying they had been misconstrued.
But his words turned out to be somewhat portentous.
At the game, a scuffle broke out between Jets and Cowboys fans. Some allege that 59-year-old Cowboys fan Leroy McKelvey was attacked for wearing a Cowboys jersey.
Others say the altercation was regarding patriotism and a refusal to stand during the National Anthem.
The fight resulted in McKelvey tasering Jets fans with a stun gun that he had sneaked into the game.
McKelvey was arrested and has pleaded not guilty to charges against him.
At Candlestick Park, several serious fights broke out between Oakland Raiders and San Francisco 49ers fans.
There was a fight in the stands; a man was assaulted in the upper-level bathrooms; a man wearing a "F**k the 49ers" shirt was shot multiple times in the stomach; and in another shooting incident, a man was shot numerous times, but according to the Los Angeles Times, he sustained lesser injuries.
And folks, this was a preseason game.
The list of hatred among rival teams seems endless
Manchester United fans and Leeds United fans despise each other.
Ditto Manchester United fans and Manchester City fans.
Ditto Manchester United fans and Liverpool fans.
Ditto Arsenal fans and Tottenham Hotspur fans.
Ditto Millwall fans and West Ham United fans.
And dozens more.
It isn't safe to wear a rival jersey to any of these games.
To illustrate, at a 2009 game at West Ham's Upton Park Stadium, fighting between West Ham and Millwall fans broke out two hours before the match. One fan was stabbed.
Fighting continued during, throughout and after the match, both in the stadium and in the surrounding neighborhood.
Bryan Stow, a 42-year-old father of two, wore a Giants jersey to Dodger Stadium in March 2011.
After the game two Dodgers fans, Louie Sanchez and Marvin Norwood, allegedly savaged him.
According to Stow's friend, Cory Maciel,
He was punched with a haymaker in the side of the head. It was a running, huge, sweeping punch. First thing that hit the ground was the back of his head, and I watched his head bounce off the asphalt, and heard it.
Stow suffered brain damage and had to be put into a medically induced coma to avoid seizures. He remained in critical condition for weeks. Eventually he improved enough to be moved to a rehab facility, though doctors believe he will be permanently disabled to some degree.
Is there something in the water at these California stadiums?
In December 2011, Sharks fans cursed and shoved a 16-year-old girl wearing a Vancouver Canucks sweatshirt.
Maggie Herger, the victim, is a diehard Canucks fan. Why? Because four years earlier, she had battled a brain tumor. While she was in the hospital, who came to visit her and cheer her up? Why, members of the Canucks.
Click here to watch an interview with Ms. Herger.
Hey, even Santa Claus isn't safe when in the midst of Philadelphia Eagles fans.
The Philly locals could teach the Inuits a thing or two about snow-packing skills.
In 1989 Eagles fans pummeled the visiting Dallas Cowboys, the officials and the television crew with snowballs.
Then in 2009, Eagles fans in the upper deck unleashed a barrage of snowy missiles on visiting San Francisco 49ers fans, helpless down below.
Click here to see the dramatic footage.
But it isn't always snowballs. Violence was so frequent during Eagles games at the old Veteran Stadiums that a criminal court, complete with a judge and a jail, was installed inside the stadium.
You might remember a little news item about some rioting at and around Rogers Arena after Game 7 of the Stanley Cup Finals last year.
It wasn't all just about looting and burning tires. Click here to see a Boston Bruins fan take a pummeling.
And there have been other incidents. Click here to watch a Vancouver Canucks fan grappling with a Anaheim Ducks fan at a game in Rogers Arena.
"The Turk Telecom Arena is a cauldron of emotion and anger that could send shivers down the spine of even the bravest fan..."—Gareth McKnight
This new stadium has replaced the old Ali Sami Yen Stadium, also known as "hell."
Think of Türk Telekom as Hell 2.0.
It is home to the Galatasaray Anonim Şirketi football club, whose fanbase ranks among the most thuggish in sportsdom.
Galatasaray fans attack rival fans and players alike.
And when annoyed enough, they've even been known to tear up their own stadium.