Paths such as the following are occurring thousands of times each month as logged by internal logs of calypso_page_view.
/stats/day/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined/undefined
Why might this be?
Just ran into this myself actually. I'm not sure what is causing it but it seems to happen every time I click on the "My Sites" button at the top left.

@psealock Where does this stand now? Have the changes been reverted? I think the fix may be relevant to the tracking issues @donpark has been working on.
Hi @yanirs I need to further investigate this comment https://github.com/Automattic/wp-calypso/pull/17583#issuecomment-325583358
I need to figure out what the difference between currentUser.visible_site_count and
sites[ blog_id ].map( s => s.visible ).length
Then I will step through the code and understand why the above comment happens and how to fix it.
Most helpful comment
Just ran into this myself actually. I'm not sure what is causing it but it seems to happen every time I click on the "My Sites" button at the top left.