apt-get update says E: Sub-process returned an error code
Solution 1:
It should also be noted that if you get a E: Sub-process returned an error code when running any something like apt-get remove [pkg] or get errors when you run apt-get update you might want to make sure your /tmp directory was not deleted.
Recreating the /tmp directory fixed my error as no application could utilize a temporary directory to store temporary files. As referenced by the last reply here http://ubuntuforums.org/showthread.php?t=1677090.
I made this answer for completeness.