Matt Simmons wrote a great post on designing elegant solutions for when users inevitably make mistakes on your system. In Engineering Infrastructures For Humans he uses the example of ash trays in airplanes to make his main point:

You don’t engineer your systems with the belief that none of your computers will ever break. That’s insane; you KNOW they’re going to break. So don’t assume that your users will never break the rules. Build in graceful failure as often as possible, whether you’re designing a user interface or a security policy.

The ash tray story is really interesting, so be sure to click through to his post.