The thing that immediately springs to my mind is making someone who has committed a perceived offense "go viral." I've seen lots of Facebook posts and Tumblr posts saying "this person did something to me that was wrong, share it and make them famous" (aka it's time for death threats and to call their employers and suggest that unsavory things happen to their mothers and children because there's no such thing as polite internet outrage.) Bonus points if you include loaded buzzwords that may or may not apply to the situation at all but will evoke strong emotions: racism, misogyny, pedophile, etc. There's lots of problems with this obviously: people act on one person's account of the story and often don't ask for the other person's - they just jump on the righteous crusade bandwagon with their pitchforks and torches. Also sometimes the person on the receiving end of the witch hunt didn't do or say anything of the sort and it's an unfortunate case of mistaken identity, revenge, or a straight up urban legend. And then there's the larger problem of mob justice. There's few to no occasions in history when a mob of angry people being judge, jury and executioner has positive outcomes.