404 page served with status code 200 (success)
https://www.dune-project.org/this-does-not-exist shows a "404 Not Found" page, but the status code returned by the web server is 200:
% wget -O/dev/null -S https://www.dune-project.org/this-does-not-exist
[...]
HTTP/1.1 200 OK
[...]
which makes various software believe the page actually exists.