If you spend time at MMA forums for any length of time you'll see people challenging others to get together and spar or even looking for people to spar with just "for practice." While most of these are just guys spouting nonsense because that's half of the reason the internet exists, there are times where the people involved are serious and it can lead to dangerous situations.

From OregonLive.com:

Investigators are looking into the death of a Chicago man who died after a two-day series of sparring matches with a Vancouver man he met in an online chat room.

Salvador Flamenco, 38, came to Vancouver on Dec. 15 after arranging the fights with the unidentified Vancouver resident through a website, www.allfighters.com, said Sgt. Scott Creager of the Vancouver Police Department. The pair decided to spar at the Vancouver man's home.

The two men fought that day and the next, but after the second day of boxing, Flamenco started complaining of dizziness and nausea, Creager said. The symptoms became more severe and by the time paramedics arrived, Flamenco had begun losing consciousness.

...

After an autopsy, the Clark County Medical Examiner's Office said Flamenco died of blunt-force injuries and ruled the death a homicide, prompting an investigation by police.

There are obvious issues with a situation like this. For one, they pretty clearly were going far too hard, far too long. Rather than going 60% in a 20 minute session it sounds like they were going 100% for long sessions on consecutive days.

I'm not really sure anything can be done to prevent people on the internet from being dumb, but hopefully enough people hear about this situation that it keeps a few from engaging in such dangerous behavior.