Ghosts

Not to be confused with Ghasts, 'Ghosting' was a phenomenon that occurred shortly after Notch added the /kill command. If one were to die or use /kill whilst wearing armor of any kind, the user would 'explode' their things, but would still technically be around. Ghosts could not be seen by other users, could not pick up dropped items, and could not be damaged. However, they are able to be perceived by monsters such as Creepers, Zombies, and Skeletons, as well as able to alter the world around them.

Notable abuses of Ghosting include teaflower's Creeper Zoo (housing at one point two or three very distressed looking Creepers), Rapier838's conquering of the Arena (illegitimately, even though he jokingly claims it was totally legit), and small hordes of Creepers seemingly following players at random.