This site's 404 not found
image is not unique. It's shared with Web Apps SE and Pro Webmasters SE.
https://webmasters.stackexchange.com/404
https://webapps.stackexchange.com/404
https://photo.stackexchange.com/404
It's entirely possible that this is intentional, but since most sites don't have the same image, I thought it was reasonable to suspect that it was a bug.
Sister bug reports:
on Meta Web Apps
on Meta Pro Webmasters