Splitting off from T130204, this is probably a separate issue.

In T130204#2130802, @ori wrote:

2016-03-17 11:39:20 mw1200 commonswiki 1.27.0-wmf.17 exception ERROR: [d1f325d3] /w/api.php?action=query&format=json&prop=stashimageinfo&siifilekey=13vted7b2nlk%2Exchznz%2E79828%2Ejpg&siiprop=url&siiurlwidth=100&siiurlheight=100   UploadStashNotLoggedInException from line 126 of /srv/mediawiki/php-1.27.0-wmf.17/includes/upload/UploadStash.php: UploadStash::getFile No user is logged in, files must belong to users {"exception_id":"d1f325d3"}
[Exception UploadStashNotLoggedInException] (/srv/mediawiki/php-1.27.0-wmf.17/includes/upload/UploadStash.php:126) UploadStash::getFile No user is logged in, files must belong to users
  #0 /srv/mediawiki/php-1.27.0-wmf.17/includes/api/ApiQueryStashImageInfo.php(57): UploadStash->getFile(string)
  #1 /srv/mediawiki/php-1.27.0-wmf.17/includes/api/ApiQuery.php(250): ApiQueryStashImageInfo->execute()
  #2 /srv/mediawiki/php-1.27.0-wmf.17/includes/api/ApiMain.php(1359): ApiQuery->execute()
  #3 /srv/mediawiki/php-1.27.0-wmf.17/includes/api/ApiMain.php(462): ApiMain->executeAction()
  #4 /srv/mediawiki/php-1.27.0-wmf.17/includes/api/ApiMain.php(433): ApiMain->executeActionWithErrorHandling()
  #5 /srv/mediawiki/php-1.27.0-wmf.17/api.php(83): ApiMain->execute()
  #6 /srv/mediawiki/w/api.php(3): include(string)
  #7 {main}

Even if there's someone not logged in requesting specific uploads from stash, which is unlikely, this shouldn't be throwing exceptions.