Seemingly I managed to get this odd data that probably shouldn't be included?
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It appears that the page-title 404.php is a valid one as per our validation scheme.
It is present in the pageview dataset on a regular basis, with spikes mainly from September 2016 to March 2017: https://pageviews.toolforge.org/?project=en.wikipedia.org&platform=all-access&agent=user&redirects=0&start=2015-07-01&end=2021-01-14&pages=404.php
The problem has been tracked and documented here: https://phabricator.wikimedia.org/T145922
