Two great examples of turning an error page into opportunity come from Wikipedia.

The genius of the wiki, as described by Ward Cunningham, comes from what happens when a page is not found. Generally the web site will simply say not found. On a wiki, the site puts up a data entry form and offers the user, the one interested in the content, to create the content. A dead-end error becomes an open door instead.

If you do put up an error message, for example when wikipedia's server fails, the outage message is also a call for monetary donations.

"Sorry- we have a problem... The wikimedia.org servers are currently overloaded, or down. Hopefully this will be fixed soon; please check back in a few minutes, as the problem is most likely temporary.... Donations Due to the ever-increasing number of people visiting Wikipedia and its sister Wikimedia projects, we have a constant need to buy new hardware to keep the site running. If you'd like to help, please donate. "