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).
Per MEE's comment, it looks like the 500 server error page and the captcha page are also serving 404 images.
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?