So far I've mostly managed to notice this with the player's not appearing to others after doing teleports. Been there for ages, as far as from beta 1.8. Players mostly appear back when you drop down a block below. Disappearing part seems to have something to do with the entitytracker, in MCP it looked like that player was not in the server's entity tracker after doing a teleport and player positions where therefore not syncronized to other players (until the player dropped a block below and then was added back into the entity tracker).
So far I've mostly managed to notice this with the player's not appearing to others after doing teleports. Been there for ages, as far as from beta 1.8. Players mostly appear back when you drop down a block below. Disappearing part seems to have something to do with the entitytracker, in MCP it looked like that player was not in the server's entity tracker after doing a teleport and player positions where therefore not syncronized to other players (until the player dropped a block below and then was added back into the entity tracker).