As is known by many, it is possible on SMP servers for the Respawn button to create multiple entities representing the player in latent connections.

Evidently, when the player disconnects, only the entity being controlled is destroyed; the other remains. However, the server does not count the player as disconnected; no "left the game" message is displayed to others.

When the player logs in again, he finds himself in the spot of the old duplicate (no matter where the controlled entity was when disconnection occurred) with an empty inventory. Other players do not receive connection notices and the player appears to have been online the whole time.