Constant teleporting between nether and oveworld (Multiplayer only)

On a SMP server, I am stuck in a pair of connected Nether portals. I constantly get teleported between the two dimensions, snapping back to the position of the portals even after moving. I also receive suffocation damage once per teleportation, and the change in the hunger bar and stack size after eating food only becomes visible after a dozen teleportations or so.

This happened while trying to go from the Nether to the Overworld; going from the Overworld to the Nether also happens. After logging out in the Nether and asking another player (who is apparently not affected) to deactivate the portals, I tried using a different pair of portals, with the same result. I eventually died of the suffocation damage in the Overworld, and after respawning went back to the Nether to collect the part of my inventory that had fallen into the portal. After collecting the items I tried to go back to the Overworld, which triggered the bug again.

Steps to reproduce:

Set up a SMP server
Give yourself the materials to build and light a Nether portal
Enter the Nether
Exit the Nether

May be dependent on network lag.

earlier versions did not happen