In an adventure map in SMP, I was experiencing strange issues where the locations of other players seemed to be offset. This would occur mainly in cramped places (on the map in question, in the dungeon at the top of the mountain).

I would notice players passing through walls, and sometimes floating or "swimming" in the ground. At some times, the offset seemed to be so bad that players would be rendered several blocks away from their true locations.

Logging out and back in would sometimes correct this issue, but it would frequently return.

I noticed this issue with:

• Players who were far away and made a long journey
• Players who were teleported to me
• Players who I was with during a long journey (namely, up the mountain in the map)

I was running the server on my own computer, and I was connected via localhost. The other players were connected from various places in different states. As my computer is not very good, I noticed a few "<Playername> moved wrongly!" messages in the console. This may be related, but I did not notice any direct connection between the messages and the phenomenon.

This issue has occurred before, on the 1.4.0 prerelease, as well, and is not an isolated incident.

EDIT: I should also note that this never happens to me on Bukkit servers, whether or not I am the host. A server host I spoke with said he has noticed this extensively and exclusively on vanilla servers when he hosts them.

Video footage can be seen in this Sethbling livestream, starting after a series of teleports at 1:38:10. Note that frequent complaints of players in the wrong position (or "invisible") have occurred throughout the video. This server is vanilla 1.4.6 (or possibly 1.4.7, but the two are interchangeable as only one class differs between the two). Sethbling may or may not be the server host; I'm sure Marc Watson of Mojang can clarify (he was present).

EDIT: Since then, I have realized this effect can be reproduced reliably on minecart entities. Steps to reproduce:

1. Run your own server
2. Set up a minecart rail loop with powered rails and a two block drop
3. Add a minecart, and make sure it goes around the loop
4. Observe that, after some time (this seems random), the minecart gets misaligned: although it tracks the same path, it may appear floating or offset by several blocks.

In my experience, lag on my system makes this effect more likely, as does moving around.