Steps to reproduce:
go in any language version to any (non existing) pages ending with .php
for example in https://wikipedia.org/anything.php (of course existing pages such as https://it.wikipedia.org/w/api.php are excluded)
if is without the /w it will always redirect to a blank page with just a tiny "File not found." If it has the /w/ the index.php will redirect to main pages

Expected results:
it should visualize the error 404 "not found" page (usually with WMF logo and black background)