On April 15th 2013, during the Boston Marathon, two home-made bombs were set off near the finish line killing 3 and injuring many others. After this, members of Reddit took it upon themselves to try and find who the bombers were. They eventually latched on to two people whom they accused and went after voraciously, including 'doxxing' (search for and publish private or identifying information about (a particular individual) on the Internet, typically with malicious intent - dictionary entry.)
Reddit accused the wrong people and the law authorities, in an attempt to calm the situation that was created, released the names of the two culprits they were attempting to arrest. Because of the early release to the media, they were able to evade arrest. The whole debacle is well documented online at this point which I would suggest you read but this post isn't about that. Rather, this is about the pitchfork mentality we find online so often.
The concept of "innocent till proven guilty" has been lost in recent years. Rather it's now guilty upon accusation, regardless of the truth. We've seen this happen multiple times where smear campaigns run rampant in the media before an investigation concludes to the point where regardless of the trial outcome, the accusation will always stick. Their name becomes synonymous with the accusation. Thanks to the power of Google, whenever you search their name you'll find those same articles accusing them.
The dangers of this pitchfork mentality are very real. In the case of the falsely accused Boston bombers, Sunil Tripathi went missing. His family was hounded by the media. A Facebook page created to find him when he went missing was closed because of the hateful comments the family were receiving on it. On April 23rd after the real suspects were finally caught, he turned up dead. The actions of redditors created some very real harassment for his family, making an already difficult time for them far worse.
That's not to say we don't need the power of communities, be they online or offline. That power was used correctly during the Arab Spring. It's currently be used to help fight for Hong Kong. But we should exercise caution and good judgement before the tools we have to fight with are taken away because of our actions.