Unfortunately I have no idea
The player would have been visible and I would have been able to actually kill him.
The player is invisible, there is no way for you to hit them, and they can kill you pretty easily.
This is a bug that exists in all game worlds as far as I understand. I’ve seen it happen in Virus, occasionally getting infected players who are invisible and basically are able to just use me as a free infect target. This seems to happen to Virus Horde zombie NPCs as well, where I get a good chunk of my health taken away by an invisible zombie before I realize what’s going on.
I believe I’ve seen it happen in Ball Race as well, where occasionally an invisible player seems to have cleared a level before I have. In any case, I believe this only happens to clients of a hosted server, but never the host itself.