Administrators can thoroughly customize every MediaWiki based wiki by editing CSS and JS files, such as MediaWiki:Common.css, MediaWiki:Common.js, MediaWiki:Vector.css etc. These customizations are usually requested by the editors community and fall into two general categories. The first category are changes to the design of the site, such as setting a different background image, a festive logo, a different font or a different color for the revision comparison (diff). The other category works around bugs until they are solved in the core code.
Customizations may need to be updated by the local site admins specially after the deployment of a new version or MediaWiki or of a customized extension. Sometimes old customizations that worked around bugs just quietly stop working after the bug is resolved. They don't affect the site much, but they fill up Common.css with now useless code that makes maintenance much harder, so it is best to remove them. Sometimes changes in the software break the customizations and the site appears broken to the users - in this case they MUST be removed or updated.
MediaWiki 1.18 was rolled out to several wikis a few days ago. Among them was mediawiki.org, the website about the software itself. MediaWiki 1.18 has many updates for the handling right-to-left (RTL) text, especially on pages where it is mixed with left-to-right text. Before the upgrade mediawiki.org had several customizations to fix the display of RTL text in the tabs on the top of the page. After the upgrade this text appeared badly until they were removed.
All the other Wikimedia wikis will be upgraded next. If you are an administrator in any Wikimedia wiki, you should look at these customization files as soon as possible and also at the release notes of MediaWiki 1.18. Understand the purpose of the existing customizations and be prepared to remove the ones that aren't needed anymore immediately after the upgrade. A general rule of thumb is to take an especially good look at lines that have "rtl", "ltr", "right", "left" and "bidi" in them.
If you need more help understanding these files, contact Amir Aharoni preferably on IRC.