A chunk with previously generated data was cleared, all the blocks and data within vanish

A chunk with previously generated data (blocks, items, mobs, etc) was cleared, all the blocks and data within vanish and replaced with default data. I have noticed this happen to me four other times before in the older versions of the game as well. (I started playing with 1.2.5)

It's very hard to notice this error happening, only when it effects the area you've chosen as a base of operations, or in a area where you would you really notice a change.

What I expected to happen was...:
The landscape, buildings, chests and the items within would remain stationary until I acted upon it to change their state. Mobs would move using there AI, but would remain with in the area.

What actually happened was...:
a chunk of data was redrawn using the game's seed as a guide. This cleared of all the blocks and mobs in the area. Some of the blocks adjacent were rewritten a few level higher than when they started.

In 2012-11-08_13.44.53.png one can see part of the building has vanished, along with the chests and their contents. You can also see part of the building redrawn on top of it's self. That "smoke stack" was not there before.

In 2012-11-08_13.45.08.png one can see the other angel of the chunk, part of this building has vanished.

In 2012-11-08_13.45.20.png same as above, in this one, the well was rewritten on top of it's self, it also capped the well (it was not capped before)

In 2012-11-08_13.46.36.png one can see the other angel of the chunk, part of this building has vanished as well as some decorations, notice the jungle tree's canopy, it was sliced off it was rounder before (as round as one could expect is a square world)

Steps to Reproduce:
I don't know how or why it happens, but this is my best guess
1. you need to have to change the area so you can see it happen.
2. load the game while you're not in the area of a chunk that has been generated previously.
3. enter that area and maybe it's changed back to the default.