looking at the sampled logs on erbium it seems we're throwing http 500s regularly when generating thumbs, most errors seem due to the wrong size being requested that would result in upscaling the image.
I'm sure this is known already but it'd seem more logical for such errors to return a 400 to the client since it isn't strictly the server's fault, what do you think?
erbium:~$ grep miss/50 /a/log/webrequest/sampled-1000.tsv.log | uniq -f2 -w16 -c | awk '{print $1 " " $4 " " $9 " " $10}' 6 2015-02-03T06:26:29 GET http://upload.wikimedia.org/wikipedia/el/thumb/9/9e/%CE%A0%CE%B1%CF%81%CE%B1%CE%BC%CE%B5%CE%BB%CE%B7%CE%BC%CE%AD%CE%BD%CE%B7_%CE%9F%CF%85%CF%81%CE%B9%CE%BA%CE%AE_%CE%B1%CF%81%CE%B8%CF%81%CE%AF%CF%84%CE%B9%CF%82_%CE%A7%CE%B5%CF%81%CE%B9%CF%8E%CE%BD.JPG/1032px-%CE%A0%CE%B1%CF%81%CE%B1%CE%BC%CE%B5%CE%BB%CE%B7%CE%BC%CE%AD%CE%BD%CE%B7_%CE%9F%CF%85%CF%81%CE%B9%CE%BA%CE%AE_%CE%B1%CF%81%CE%B8%CF%81%CE%AF%CF%84%CE%B9%CF%82_%CE%A7%CE%B5%CF%81%CE%B9%CF%8E%CE%BD.JPG 19 2015-02-03T06:30:14 GET http://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Commons-logo.svg/36px-Commons-logo.svg.png%202x"%20data-file-width="1024"%20data-file-height="1376"%20/></a>%20<b><span%20class="plainlinks"><a%20class="external%20text"%20href="http:/lookup-api.apple.com.edgesuite.net/commons.wikimedia.org/wiki/?uselang=it">Commons</a></span></b> 7 2015-02-03T06:40:19 GET http://upload.wikimedia.org/wikipedia/en/thumb/6/6f/Vathikuchi.jpg/402px-Vathikuchi.jpg 10 2015-02-03T06:51:42 GET http://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Valencia01_11-2-03.JPG/794px-Valencia01_11-2-03.JPG 7 2015-02-03T07:00:14 GET http://upload.wikimedia.org/wikipedia/en/thumb/9/9e/Koli_tree_near_Panamaram_fort_site.JPG/1024px-Koli_tree_near_Panamaram_fort_site.JPG 13 2015-02-03T07:10:25 GET http://upload.wikimedia.org/wikipedia/ja/thumb/9/95/%E3%83%88%E3%82%B3%E3%82%B8%E3%83%A9%E3%83%9F_%E6%8A%9C%E3%81%91%E6%AE%BB.JPG/640px-%E3%83%88%E3%82%B3%E3%82%B8%E3%83%A9%E3%83%9F_%E6%8A%9C%E3%81%91%E6%AE%BB.JPG 14 2015-02-03T07:21:22 GET http://upload.wikimedia.org/wikipedia/commons/thumb/6/61/Opentail_g.svg/8px-Opentail_g.svg.png"%20width="8"%20height="14"%20srcset="http:/lookup-api.apple.com.edgesuite.net/upload.wikimedia.org/wikipedia/commons/thumb/6/61/Opentail_g.svg/12px-Opentail_g.svg.png%201.5x,%20http:/lookup-api.apple.com.edgesuite.net/upload.wikimedia.org/wikipedia/commons/thumb/6/61/Opentail_g.svg/16px-Opentail_g.svg.png%202x"%20data-file-width="115"%20data-file-height="196"%20/></a> 10 2015-02-03T07:30:50 GET http://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Flag_of_the_People%27s_Republic_of_China.svg/35px-Flag_of_the_People%27s_Republic_of_China.svg.png%201.5x,%20http:/lookup-api.apple.com.edgesuite.net/upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Flag_of_the_People%27s_Republic_of_China.svg/45px-Flag_of_the_People%27s_Republic_of_China.svg.png%202x"%20data-file-width="900"%20data-file-height="600"%20/>& 11 2015-02-03T07:40:05 GET http://upload.wikimedia.org/wikipedia/commons/thumb/2/23/Casino_slots2.jpg/ 18 2015-02-03T07:50:32 GET http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Laos.Visa.JPG/1005px-Laos.Visa.JPG 15 2015-02-03T08:00:36 GET http://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Fairytale_bookmark_silver.svg/21px-Fairytale_bookmark_silver.svg.png%201.5x,%20http:/lookup-api.apple.com.edgesuite.net/upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Fairytale_bookmark_silver.svg/28px-Fairytale_bookmark_silver.svg.png%202x"%20data-file-width="100"%20data-file-height="100"%20/></span>%20<i><a%20href="/wiki/L%27Illusion_comique_(t%C3%A9l%C3%A9film)"%20title="L 9 2015-02-03T08:11:23 GET http://upload.wikimedia.org/wikipedia/commons/thumb/8/88/WIN31SOD.JPG/816px-WIN31SOD.JPG 11 2015-02-03T08:20:25 GET http://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Warszawa%2C_Pa%C5%82ac_Kultury_i_Nauki_-_26.11.2009_r..JPG/728px-Warszawa%2C_Pa%C5%82ac_Kultury_i_Nauki_-_26.11.2009_r..JPG 15 2015-02-03T08:30:00 GET http://upload.wikimedia.org/wikipedia/commons/thumb/6/66/Torre_de_Belem_1.JPG/1024px-Torre_de_Belem_1.JPG 20 2015-02-03T08:40:03 GET http://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/TRON_9-9145.gif/16px-TRON_9-9145.gif 22 2015-02-03T08:50:08 GET http://it.wikipedia.org/w/index.php?title=Speciale:Captcha/image&wpCaptchaId=160756292 18 2015-02-03T09:01:52 GET http://upload.wikimedia.org/wikipedia/en/thumb/6/6f/Vathikuchi.jpg/402px-Vathikuchi.jpg 23 2015-02-03T09:11:18 GET http://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/2005_Pebble_Beach_Concours_Bertone.PNG/466px-2005_Pebble_Beach_Concours_Bertone.PNG 19 2015-02-03T09:20:28 GET http://upload.wikimedia.org/wikipedia/en/thumb/4/49/ImamRezaHospital.JPG/800px-ImamRezaHospital.JPG 10 2015-02-03T09:30:18 GET http://upload.wikimedia.org/wikipedia/en/thumb/2/2e/Wine_TV.PNG/96px-Wine_TV.PNG