.htaccess contains
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteRule ^ index.php [L]
which effectively means that Apache won't run PHP and therefore no Drupal bootstrap happens. Apache handles the request itself. However, what that means is that Apache's own error log will be filled with entries about a missing 'favicon.ico' file. This is what gagarine mentioned in #174940-69: Useless favicon.ico 404 messages in logs → and this has never been fixed.
Credit to a.ross in #121 🐛 Root /favicon.ico accounts for majority of 'Page not found' errors Fixed
Review
Fixed
8.9 ⚰️
Last updated
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
My Drupal 7 logs were getting filled with "page not found" entries looking for a root favicon file at https://example.org/favicon.ico. I thought this issue was fixed by default in Drupal 7, but I had to enable drupal_fast_404(); in settings.php and now these requests are not getting logged, great.