Speaking of unintended consequences, some clever webmasters have crafted their own ‘fail’ pages, aka ‘Page Not Found’ or ‘404 Errors’ (thanks Susanne). The most famous, of course, is Twitter’s iconic ‘Fail Whale’.
It’s bad enough that the website hasn’t delivered what the user wanted, but rather than just letting a banal, technical page appear, these folks have embraced even the failure scenario as a way to express their individual personality and deliver a bit of light fun to the user at the very least.
Leadership and Management / Turning Adversity to Advantage 
Failure Magazine has a great interview with Yiying Lu, the designer of Twitter’s ‘Fail Whale’ – http://failuremag.com/index.php/feature/article/yiying_lu_the_failure_interview/
And perhaps the ultimate 404 (thanks Steve) http://www.masswerk.at/404 (hint – ‘space’-bar and enter key).