Issue with blocks appearing to be something they are not, aka "ghost blocks".
Best described with a video (schematic download in description):
http://youtu.be/E4zr0sEJ7Wo

Crash dump:
https://gist.github.com/4042984

This happens on vanilla version 1.4.4, 1.4.3 & 1.4.2 with default textures.