The ProofreadPage extension seems to be manually constructing thumb URLs and assuming it can slap in any old number. This fails when, for instance, we have disabled scaling of GIF images:
var proofreadPageThumbURL = "http://upload.wikimedia.org/wikisource/en/thumb/1/14/Summary_for_B-23.gif/WIDTHpx-Summary_for_B-23.gif";
Resulting files fail to render:
http://upload.wikimedia.org/wikisource/en/thumb/1/14/Summary_for_B-23.gif/475px-Summary_for_B-23.gif
and so you're left with a big blank spot.
ProofreadPage's JavaScript should use the API to look up thumb URLs if it's necessary to dynamically size them.
Version: unspecified
Severity: enhancement
URL: http://en.wikisource.org/wiki/Page:Summary_for_B-23.gif