One that Apple may have blocked for security reasons, currently likely to be Flash Player related.
Two bugs, one affecting Apple's Mac platform and another attacking Microsoft's Windows, exploit certain Flash player vulnerabilities to install malware onto users' systems, reports ArsTechnica. While users of other operating systems like Linux have yet to report attacks, Adobe's advisory notes the exploit affects all platforms.
Designated as CVE-2013-0634, the first vulnerability targets the Safari and Firefox Web browsers running on OS X, and is also being used as a trojan to deploy Microsoft Word documents containing malware. For Mac users, the flaw affects Adobe Flash Player version 11.5.502.146 or earlier.
On March 1 Apple again blocked Flash Player for Lion and Mountain Lion:
You should uninstall any previous version first, and repair permissions after installing the new version.