In my screen I should have 5 columns and 4 rows in a total of 20 thumbnails. But as you can see from my screenshots, sometimes it's short a few thumbnails.
As i've observed, the problem randomly occurs in post pages. And waiting a few hours and checking the same page again doesn't seem to yield the same results. I've tried it last night and checked today and saw that the pages with the missing thumbnails are now different. Maybe it has something to do with new uploads.
I've already tried clearing my browser's cache just to be sure.
screenshots:
link to page: http://danbooru.donmai.us/posts?tags=original
screencap: http://i.imgur.com/6Bnn1UJ.png
link to page: http://danbooru.donmai.us/posts?page=5&tags=original
screencap: http://i.imgur.com/ff7ML5K.png
link to page: http://danbooru.donmai.us/posts?page=7&tags=original
screencap: http://i.imgur.com/DGl5tZK.png
link to page: http://danbooru.donmai.us/posts?page=9&tags=original
screencap: http://i.imgur.com/mJKLEFH.png
link to page: http://danbooru.donmai.us/posts?page=10&tags=original
screencap: http://i.imgur.com/HmFM0U3.jpg
Updated by Toks