Sort of extreme chunk errors where random stuff, such as end portal frames are generated in huge veins throughout the Nether. ALso caused chunk errors in The End, as well as massive lag.

What I expected to happen goes here...
The Nether/End generates perfectly, with minimal lag.

What actually happened was...:
Large veins of random stuff generated throughout the Nether, causing massive lag, as well as causing chunk errors, weird generation, and a "vibrating" enderdragon in The End

Steps to Reproduce:
1. Download and install snapshot 12w49a
2. Enter Nether or End
3. Observe