There was a problem with the image http://commons.wikimedia.org/wiki/File:Haiti_topographic_map-fr.svg, where the font used everywhere was DejaVu Sans Condensed, and the font DejaVu Sans was used for the legend of the altitudes (the leftmost box).
As you can see in the "file history" on this page, on the bottom thumbnail, the thumbnail rendered incorrectly, with the text in DejaVu Sans creating large black areas, going outside of the box.
Changing the font to DejaVu Sans Condensed solved the problem, as can be seen again in the file history, the top thumbnail.
Maybe it is a problem with font scaling? This should probably be addressed as DejaVu Sans is a fairly standard font, used now for replacing Arial.