Minecraft 1.7.2, Minecraft 1.7.9, Minecraft 14w34d, Minecraft 1.8-pre3, Minecraft 1.8, Minecraft 1.8.1, Minecraft 1.8.3, Minecraft 1.8.7, Minecraft 1.8.8, Minecraft 15w34d, Minecraft 15w40b
Windows 7 Home Premium x86
Intel Celeron C900 (single core not too good)
Player lagged out (timeout due to bad connection), as a result the server didn't kick the player out of the game. Player rejoined and ended up spawning inside the old player position which then turned into a ghost. The real player can move around normally, but the ghost simply stands in place, but is able to take damage as if it were a normal player. The damage doesn't affect the player the ghost came from. After about a minute the ghost will vanish and the server will say the player left the game, but /list shows the real player and other players like normal.
In our occurrence, we were getting the "Diamonds to You!" achievement, one player threw the diamonds to me (8 of them), on my screen I threw them back. Then I lagged out, rejoined, and I still had the 8 diamonds, however the player I returned them to retrieved them as well, and had tried to send 1 back to me again (since I threw all out at once rather than simply using the Drop Item key), which may have been collected by the ghost player. So we ended up with 15 diamonds through the lag, and these diamonds were able to be moved around and crafted with without any disappearing (as if it were a client error).