Comment 3 for bug 693442

This problem still occurred whenever I used apt to install new software (via Terminal, Synaptic, and Software Center). Every package gave me the same error. I got impatient and ran this:

sudo rm -f /etc/gshadow.lock /etc/shadow.lock /etc/passwd.lock

I no longer get any errors upon installing new packages. Completely solved my problem, even after several reboots. I'd still like to understand why that was necessary though.