A redditor made a gif Trump hitting CNN with a chair in a wrestling ring, or something to that effect. It was subsequently tweeted by Trump. CNN tracked the gif back to the redditor that made the gif and threatened to go public with his identity and reddit comment history if he didn't issue an apology (which he did).
I think the lesson is something about picking fights with people who buy ink by the barrel.
It was investigative journalism, if you can call it journalism. CNN investigated the origin of the gif, and used biographical details revealed on reddit to determine the gif creators real name / identity. They didn't ruin his life, but threatened to go public with information which probably would have.