I was a lab assistant for a linux class. One of the students, called me over because she could no longer `su -` because she was getting permission denied. OK, she's misremembered/mistyped the password. Reboot into single-user mode and reset. What?! `su` STILL doesn't work?! It MUST bow to my will! So I reboot into single user mode to find out what she did. I realized that she ran `chmod -R 777 /var/www/html/drupal-6.19 /`

Note the space between the directory name and the final slash.

After a few minutes of "I really don't want to have her reinstall, so what is this doing and how.", I managed to find that /bin/su now had file permissions of 777. That can also be read as file permissions of 0777, which removes the setuid bit from /bin/su. A quick `chmod u+s /bin/su` and I was a hero.