With the clock nearing zero, the 2006 divisional round playoff between the Indianapolis Colts and the Pittsburgh Steelers looked to be over: It was the fourth quarter, with one minute and 20 seconds left, and the score was 21-18. Pittsburgh held the lead and, by all appearances, was about to score again.

Pittsburgh's offense lined up on the Indianapolis 2-yard line and handed the ball to future Hall of Fame running back Jerome Bettis, a cannonball of a man who famously went by the nickname "The Bus." Nearly everybody assumed Bettis would pound the ball through the goal line. Instead, Colts linebacker Gary Brackett forced a fumble. The Colts picked up the ball and nearly ran it back for a touchdown. For Steelers fans, it was a sudden and heartbreaking turn of events. Literally.

Watching from a bar, a diehard Steelers fan named Terry O'Neill watched the ball tumble to the ground and suddenly felt a pain in his chest. Luckily, two firefighters in the crowd helped resuscitate him.

"My heart just quit beating completely," O'Neill later told the South Pittsburgh Reporter. "For all intents and purposes, I died."

Research indicates he wasn't the first. Watching a high-stakes game could actually kill you.

A 2002 study in The BMJ, which focused on the health of English soccer fans, found that a "myocardial infarction can be triggered by emotional upset, such as watching your football team lose an important match." A 2008 study published in the New England Journal of Medicine followed the World Cup-watching habits of German soccer fans and found that watching a stressful game more than doubled viewers' chances of experiencing a cardiovascular event. A similar result was found when other researchers looked at cardiovascular deaths in the Netherlands after the country's soccer team lost the European soccer championships on a penalty shootout in 1996.

In 2011, a study published in Clinical Cardiology looked at the Super Bowl specifically and found that deaths increased after the big game in the losing city, finding an "absolute increase in all cause mortality" in people over the age of 65. The researchers argued:

"Acute risk factors usually involve some form of stress—physical, emotional, or both—that increase the sympathetic nervous system and releases catecholamines. The subsequent increase in heart rate, blood pressure, and ventricular contractility increase oxygen demand and may change the shear stress of blood against an atherosclerotic plaque, contributing to plaque fracture."

This particular study, however, has received some criticism. It only looked at mortality statistics for the 1980 and the 1984 Super Bowls, a relatively small sample. Some researchers said the study went too far in implying that the Super Bowl caused death, considering that the viewer's behavior and health history (and not the events of the game itself) could have been responsible. Super Bowl Sunday, after all, is a day filled with fatty fried foods and copious amounts of alcohol—all possible risk factors for a cardiovascular event.

As Gregg Fonarow, director of the Cardiomyopathy Center at UCLA, tells LiveScience, "It may be other behaviors associated with important sporting events rather than the stress of watching the home team lose that may explain these associations." Additionally, pre-existing conditions could be a huge contributing factor. (This was the case for our fateful Steelers fan.)

Study limitations aside, becoming invested in the outcome of a sporting match is undeniably stressful on the heart. A recent (though small) study out of Canada surveyed the heart rates of hockey fans during games, revealing "a mean increase of 92 per cent among the 20 test subjects, rising from an average rate of 60 to 114 beats per minute," according to the Montreal Gazette. In other words, people sitting and watching TV had heart rates equivalent to people undergoing mild exercise. Their heart rates only got higher when they watched games in person.

Of course, you don't have to do a study to learn that close games can cause a diehard fan's heart to pound—just go and ask one. And if they mutter, "This team is going to kill me!," kindly suggest that they step away from the TV before it becomes a self-fulfilling prophecy.