For the non-techies, to a web server a 404 error is a “page not found” error. You typed www.cutekittens.com/extracute but that’s not a real page.
In the olden days, you would get a generic web server page. But as tech folks realized that this was a common occurrence, they started developing custom error pages that were at least amusing.
I like this one, both for humor and realism:
Here are several good ones…