This was one that took place online. So I was on this site called Nationstates, basically a country simulator. I was the founder and leader of a "region" which in game is a collection of "nations" or players. Two guys, one of them a friend I'd known for a long time, started fighting nonstop and I ended up kicking my friend from the region because I was convinced he was the one who was in the wrong. He was one of my closest friends but we grew apart after arguing over him fighting with the other guy. I realized too late that I banned the wrong guy and now I've lost a good friend. Permanently.