Yeah I noticed this earlier as well
This was a transient issue whilst some changes to the popularity system were being made (you were seeing posts without scores yet at the top rather than the bottom) - it should all be definitively fixed as of some hours ago.
Still not fixed for me. Every day/week/month from 4/14/2013 on still has posts with zero votes and public favorites as the most popular images.
http://chan.sankakucomplex.com/post/popular_by_day?day=14&month=4&year=2013
date: + order:popular works, but is significantly clunkier.
Same here. Still not fixed.
Apologies - wasn't aware it was the popular_by_* which were at fault. That should be fixed as well now.
Confirmed. Cheers System :)

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someguy1653
1 year agosomething screwy is going on with popular images
suddenly within the past couple of hours the most popular images became low quality images from the most prolific bots. These images have no votes and no public favorites. We are taking about posts minutes old to months old suddenly becoming the most popular images in their time frame. This is also probably why the site is slow to load pages now.
edit: It seems to start in 4/14/2013 and includes hundreds of images every day from then to now.
I think the code itself to determine the popular images was corrupted. A one minute old image instantly becomes the most popular image for April 2014, only the be replaced by the next image posted.