I edited /etc/sudoers without visudo, and made a mistake. That will prevent you from successfully using sudo again. No real harm done, but it takes rebooting to get it fixed.
1) reboot in recovery mode (press escape when booting so the grub options are shown)
2) drop to a root shell (option in the recovery menu)
3) mount -o rw,remount /
4) visudo (emacs based editor)
5) reboot the system
You should be up and running again!