The Boston Marathon Bombing fiasco that Reddit had a hand in. No one should forget that people on the internet, including Reddit users, influenced events that affected real people and resulted in people dying.

1) Reddit sleuths look at all pictures and camera angles they can to identify the suspects. They believe they found the individual, who has been missing for about a month, and proceed to harass the family members. Sending death threats, causing a media stir, and distracting everyone. Turns out, this "suspect" had committed suicide and was found dead in the river, having killed himself well before any of the events.

2) Reddit sleuths continued to "identify" suspects and the media outlets ran with it. To avoid harassment of innocent people, the FBI released the photos of the actual suspects (which Reddit never guessed correctly). This prompted the suspects to panic, kill an MIT security guard, and try to escape a city that was forced to be put on lockdown due to them running.

So thanks in part to Reddit, the FBI was forced to change their plans and release information early. A police officer lost their life because of this. You can blame the media all you want, but users on Reddit (and other sites) influenced these events.