FOSS gamedev and creative worlds

When I just loaded my singleplayer world, Many of the things I have built have sections missing.
In fact, One of my Houses are missing! An Entire House!!!
Along with that, some blocks have been replaced.
I would never do this, even some of my larger buildings have entire walls missing.
Heres My list of missing or replaced....things

Missing:
An Entire house
The Entire back section of my City Hall
Three different walls, steps, and the wood floors of my Tavern, and my bathroom, for some reason....
The Wood floors and steps of my School House
Some Items I placed in a couple chests
Some wood blocks are missing
My City Pool is full of dirt, only one Block deep of water, it was cobble sides, and smooth stone on the bottom, and even had some steps to get out, and my diving board is missing a block
Some dirt pathways that I dug out are filled in... again
And my mine was filled in.

Replaced:
Some wood blocks that I used in my floors were replaced with dirt...bloody dirt (and yes, that was a reference to the Yogscast)
And some cobble was replaced with, you guessed it, dirt.
I will post some pics ASAP
The Version I use is 0.4 dev 20120606
I have Windows

I also just realized the version im using isnt stable yet......

It's called ''world destabilization''. It usually happens when you try to re-mapgen parts of the land (IDK how..? maybe loading the map the wrong way?) or when the server crashes while the map is being edited, leaving ''holes'' in the data that cause the map to be re-terraformed. Most likely it's the first problem, as the second one also crashes the game when you enter the affected areas.