It looks like the image that is supposed to show up in the 404 error page has...itself gone 404, and shows a broken image.
It looks like it's trying to load the image from https://cdn.sstatic.net/Sites/codegolf/img/404.png, but that link is 404 (as of right now).
Can this please be fixed? (Also, can someone please create the 404-not-found tag on this site, as it doesn't exist, and there don't seem to be any relevant tags other than the required tag to use?)
Update: It seems that the broken image was manually resolved by changing the URL of the loaded image to the same one used on beta and non-designed sites, i.e. https://cdn.sstatic.net/Sites/beta/img/404.png. However, the site-specific 404 image, linked above, is still broken. Is this site not going to get a uniquely-designed 404 error page as part of its design?