I have just discovered a great WordPress plugin that significantly reduces the chance of a user ever seeing a 404 error page.
When a page cannot be found, Smart 404 will use the current URL to attempt to find a matching page, and redirect to it automatically. Smart 404 also supplies template tags which provide a list of suggestions, for use on a 404.php template page if a matching post can’t be immediately discovered.
What a superb idea! Try it out on this post. Go to the address bar and delete the word smart from the URL. It will still find this page.