Development console output and crash report attached. At the time of the crash, lag was pretty bad - I swung at a skeleton (both of us not moving) several times before it started registering hits. A stone block had done the re-appearing trick just before that. The sound of torches being placed was lagging before that as well.
I also lost all achievements when I auto-updated to 1.7.2 from 1.6.4 on my Mac bookj running OS X 10.8.5. Note: I had previously encountered the bug where "The End?" was never granted, although I had passed through the portal (and later killed the Ender Dragon).
How did it get corrupted, and is there any way to un-corrupt it?