Hi Kalena,
Yes, indeed those 404 pages are very refreshing and are bubbling with creativity, hovever i’ve noticed that some of the 404 pages are not actually ‘true’ 404 error pages. While checking the server response for some of the pages, i’ve found out that some are returning a header 200 instead of a 404 !
(e.g: http://www.fuelly.com/404.html)
Well, in my opinion i believe that having an error page with header 200 can mess up search engines and possibly result in a whole bunch of indexed URL’s all looking to the search engine like duplicates of web-pages. In fact, the search engines will think that all is well and that there’s no error at all since it is reading header 200, while it should be reading header 404 and understand that this page (and URL) must be removed from its database.
Furthermore, site logs will not report errors if it responds as a 200, and also will not be verified if we dont have a ‘true’ 404 error page. (While using Google Sitemaps for e.g)
In brief, the 404 page should have server response 404 – not found. That’s the whole purpose of the page, to let search engines understand that this page no longer exist. Then of course we can add some creativity to the page to make it fun for visitors…