Permissions fixing tool

Summary:
An application or utility--even a command-line tool--to repair permissions on a broken Ubuntu system.

Rationale:
Sometimes new users get overzealous and chown -R or chmod -R their whole systems in a bad way (to user or 777, respectively). Sometimes even experienced users make a typo in a command and end up screwing up permissions or ownership on an entire directory.

Scope and Use Cases:
Lena accidentally changed permission on her entire Ubuntu installation to 777. Now she has a totally non-functional Ubuntu. The best advice she gets from other forum users is to reinstall Ubuntu.

Implementation Plan:
While there's no way to know what permissions and ownership personal files could have had, there is probably some kind of template or comparison a program could make on important system files (like /etc/sudoers) and general user files (like .dmrc). If the system is mostly borked, maybe it could be a simple command you run from Recovery Mode
Code:

sudo permrepair

If the system is completely borked, maybe it could be a few commands you run off a live CD.

This may be technically too difficult to implement, but I thought I'd just throw it out there as a suggestion.