The Knockout Game is a controversial video fad that involves punching an unsuspecting bystander in the face, often with the intention of knocking the victim out unconscious in a single blow.
The earliest known incident of violence resulting from playing the so-called "Knockout Game" took place in St. Louis, Missouri on April 16th, 2011, when the then-18-year-old Elex Levell Murphy fatally punched 72-year-old Vietnamese immigrant Hoang Nguyen to his death. According to the police investigation, the attack was revealed to be part of a dangerous game "which involves unprovoked attacks on innocent bystanders."
Precursor: MTV Jackass
A running gag in the 2010 comedy film Jackass 3D featured cast and crew members being surprised with a hit in the face with a boxing glove by actor Bam Margera (shown below).
In the following years, several murder and manslaughter cases with ties to the knockout game have been reported in the U.S. news media, including the deaths of a 20-year-old college student in St. Cloud Minnesota, a 51-year-old man in Syracuse, New York, a 46-year-old man in Hoboken, New Jersey.
Meanwhile, amateur-recorded video footage of incidental knockouts during real-life street brawls also began circulating on video-sharing sites like LiveLeak and World Star HipHop (shown below).
Smack Cam Vines
During the summer of 2013, a similar game derived from the dangerous fad began spreading across Vine under the tag "Smack Cam," which involves slapping an unsuspecting victim in the face and capturing the moment on camera. On June 27th, the earliest known instance of "Smack Cam" clips was tweeted by Vine user Max Jerry, receiving more than 175 retweets in the first month.
On July 10th, 2013, Max Jerry uploaded a compilation of his "Smack Cam" Vine videos to YouTube, gaining more than 291,000 views and 580 comments in the next three weeks.
On July 26th, the women's interest blog Jezebel published an article criticizing the fad, which linked to several Smack Cam Vines featuring women being violently hit in the face, many of which have since been deleted. The same day, the pop culture blog Complex published an article about the videos, which quoted Smack Cam creator Max Jerry's opinion on how the fad had become increasingly violent.
"I'm really scared how far people are going with Smack Cams now. When boys are smacking girls, hitting animals or even kids, it's a bit out of hand, especially with hard objects. I started Smack Cam for fun, not to see people come out with serious injuries."
Also on July 26th, The Atlantic Wire and the Daily Mail reported on the controversy surrounding the videos. On July 28th, The Huffington Post published an article about the fad, which highlighted several Vine video examples.
 River Front Times – Elex Murphy: St. Louis Teen Allegedly Killed Elderly Immigrant As Part Of Game
 St. Louis Post-Dispatch – 'Knockout game' widow tells of lonely life after husband's murder