I had a bit of a scare with a possibly corrupted sudoers file this evening, so this is just a reminder to always edit your sudoers with:
visudo

Sure, just issuing a:
nano /etc/sudoers

Will probably do the same thing for you, but one nice thing that visudo does for you is it checks the file for any problems when you try to save it, which could possibly prevent you from breaking your system like I did.