If you go to http://3e.org/dmd/scents/nosuchpage you're treated to a "404 not found" page, as you rightly should be - that page doesn't exist.
However, the page is served up with a status code of 200 !
Why this can be a problem: I just moved from MovableType to WordPress. All my old URLs are now invalid, but Google will keep crawling them for a long time. If Google were to get a "404" when it tried to access the old URLs, it would drop them from the cache and stop coming back.
Instead, search engines are being told by WordPress that those URLs are still valid pages ... pages that happen to contain the text "404 page not found ... Apologies. There's been a problem finding the page you're looking for. ".