Today I wanted to install some mkv tools via macports and received strange response:

$ sudo port install mkvtoolnix
sudo: can't open /private/etc/sudoers: Permission denied
sudo: no valid sudoers sources found, quitting

After running $ ls -lad it turned out that something is wrong with boot volume permissions. Theese are corect ones:

$ ls -lad / /private /private/etc
d--xrwxr-t+  36 root  wheel  1292 25 sty 12:34 /

This problem cannot be solved by simple Repair Permissions in Disk Utility. The only way to solve it is booting your computer in Single-User Mode (hold cmd+s while booting) and entering theese commands:

# /sbin/fsck -fy
# /sbin/mount -wu /
# /bin/chmod 1775 /
# /bin/sync
# exit

Everything should work now. :)