I read the snippet about update 1.6.4 and about how I should load worlds with it before I play with 1.7. I updated and created a world, then x-ed out and loaded up Minecraft again, except with snapshot 13w38c. I got into the world I had made and I was burried in dirt. I was in an ocean before and now i was swimming in dirt (and drowning in it XD). I got out of that and looked around and there were huge chunk errors in many places. I tested this with a few other worlds (create in 1.6.4 and then play in 13w38c) and it seemed to have the same effect, though not always did the chunk errors happen near. Some happenned at the very edge of the loaded chunks i had loaded in 1.6.4. This may just be from changing between them but still needs to be fixed.