I've noticed that Nether Quartz Ore started appearing with 13w10a in my world. I have a rail going to a far Nether fortress and the ore appeared under it as well. In few blocks distance everything I created or changed was reverted back like I've never been there before. Also, HDD went crazy, system and minecraft hang. Ghast was stuck in the ground near there as well.

I tried to fix it by rebooting computer / restarting minecraft server a few times but had no luck.
I reverted back to 13w09c but the damage was already done.

While I had the Nether problem, the normal world was completely fine

Fortunately, I create backups every day so I could replace the damaged one and now everything's fine on 13w09c.

Sorry, guys I'm new here. Not really sure if this is the correct way of reporting a bug. I'm happy to answer any questions or create screenshots in the undamaged world if needed.