The story I'm about to tell you is very much true, and the memory still haunts me to this day. I had loaded up Half-Life 2: Deathmatch one day, and went on a server called Antalink. Only one person on, which I kind of wanted anyway. I go in, and I spend some time looking for the player who was on. I found him, but he was hitting a wall like something was there and he didn't like it. I killed him, but when he respawned he resumed his previous task. I tried talking to him through the server chat and on microphone, but got no acknowledgement or reply. I was a bit pissed since I had encountered something like this before. I let him do his shit.
After a while the crowbar hitting concrete got annoying and I decided to follow him to see if he was trying to point something out to me. He even accidentally hit me, but didn't acknowledge he did so. I didn't shoot since I realized it was an accident. I followed him throughout the map, and he stopped at a corner and continued hitting, but of course nothing was happening. I decided to walk away to collect some ammo. At one point as I was walking across a catwalk I saw him at a suit charger, but he wasn't using it. Just looking at it. I threw a grenade down, and he didn't even run from it. Boom.
This kinda went on for several more minutes, and then some other players joined. I thought they would talk some sense into this crazy motherfucker. But they didn't. In fact, they didn't seem to even acknowledge him. If they killed him it was an attempt to kill someone else. I tried saying something, but no one gave a reply. Everyone was too busy with something. Finally, the map changed. Ten seconds after everyone got into the new map, the ghost player left. Ten minutes into the map, he returned. He did some killing, but no one but me fought back against him. I kicked his ass, and he quit again. He never returned after that.