Last modified: 2011-07-08 15:36:06 UTC
When mathematical expressions contain a fraction stroke within them, then their background is white, not transparent. This usually isn't a problem as MediaWiki's default background is white, too, but in certain cases (e.g. within wikitables), the white background is clearly visible and looks very ugly. I'll post a link with an example.
This is fixed, you're just looking at cached math images from before the bug was fixed. Marking as dupe of bug 24445, but could also be called a dupe of bug 16719 *** This bug has been marked as a duplicate of bug 24445 ***