Contingency Design
37Signals have an interesting whitepaper regarding Contingency Design: Maximizing Online Profitability By Helping People When Things Go Wrong which is a 17 page pdf which is available for download.
I think that it's important to utilise certain tricks when viewing sites and you happen to mistype a URL and cause a 404 error. For example IS's and UUnet's websites both return 404 errors while my website displays a page saying there was a 404 error with navigation. The PHP website redirects you to a search page.
Contingency design is design for when things go wrong. It's the error messaging, graphic design, instructive text, information architecture, backend system, and customer service that helps visitors get back on track after a problem occurs.