While playing Minecraft over LAN using the server, chunks in which we already had built something have been reset to the way they were when we created the world (so everything we built was replaced by a big Block of stone, earth, dirt and air).
(see screenshots)
We had just updated the server to the new version. Me and my Friend had been mining in caves before the update, however then decided to teleport us back to our base (a castle we had built somewhere where there was originally a mountain). Arriving there, there seems to have been an error loading the chunks causing the problem said.

What I expected to happen was...:
I expected to see the castle we had build the way it was before.

What actually happened was...:
Large parts of the castle (propably chunks) have been replaced by the terrain that had been generated there when we originaly created the world.

Steps to Reproduce:
We haven't had that bug since, but what we did was this:
1. Update Minecraft client from 1.6.2 to 1.6.4
2. Update Minecraft server from 1.6.2 to 1.6.4
3. Connect to server.
4. After three minutes at most, teleport to a location at least 500 blocks away, where you have already built something.
5. Some Chunks have been replaced by the terrain that was generated when the world was created.