404, error, pages are an aspect of a site design that can easily be forgotten. Visitors are very unlikely to get to see a 404 page if all links are active, but there will be occassions, when they will be called into action.
The lack of an error page is not the end of the world, but it does suggest that the creator of the site hasn’t thought of everything, which may lead users to lose trust in the site. Default error pages are at best adequate, but lack personality and interest. A good 404 page should engage the viewer and be functional, eg inlcude links to key pages, or at the very least the home page. Check out these five examples