The margin around floated thumbnails is forced to be while.
Especially when used in an element with toccolours or when used in the {{Information}} template on Commons, this gives an ugly edge.
See also:
http://www.mediawiki.org/w/index.php?title=Sandbox&oldid=371897
Assigning to myself, I've fixed this on a few local wikis and if I see no collisions, I'll put it in core as this is a general MediaWiki bug and not specific to Wikimedia.
Version: 1.18.x
Severity: normal
URL: http://translatewiki.net/w/i.php?title=Sandbox&oldid=2584255