Note: you should always 'sudo visudo' to edit the sudoers file, it validates the file and thus will prevent you ending up in this situation in the first place!
So I went to edit /etc/sudoers, and mistakenly assumed that the '#' in front of the includedir directive line was a comment. It is not.
So now, if you're silly like me and you weren't making changes as root, you can't sudo to fix your sudo. Oh the irony.
You'll need to:
- Reboot into recovery mode (hit escape during boot, choose the recovery mode option on the grub screen)
- Choose the 'Enable networking' option (if you don't your filesystem will be mounted as read-only. who knew)
- Chosee the 'Drop to root shell' option
- run visudo, fix your file
- Reboot with normal grub option