A guy I knew came on my private server when we had a bunch of other people on. He asked for a few of his friends to be whitelisted, but I was a bit reluctant because I didn't know them. It turns out they were part of his griefing team and they decided to grief the spawn, the village, a nice house I had just finished, and post signs saying nasty stuff everywhere. He also lied about one of his friends, saying I knew him and that he was in our Scout troop so I would whitelist him. Didn't have a anti-grief plugin at the time so I had to rebuild everything by hand D: