Symantec is again in hot water over an update. This one is localized in nature, with an update to their anti-virus suite mis-identifying files in the Simplified Chinese version of Windows XP. Ultimately, the AV suite quarantines critical files, resulting in a crippled system. It only affects that one particular version of Windows, though that is still a huge problem for those in China or other countries using that localization. It also apparently only affects systems that have a particular patch applied through Windows update. That aside, it seems to be a big issue:

CISRT said the flawed Symantec update only affects users of the Simplified Chinese version of Windows XP Service Pack 2 that have been patched with a particular Microsoft software fix available since November 2006. CISRT noted that this issue has been "huge."
To their credit, it took Symantec only four hours after being notified of the issue to correct their patterns and make new ones available via LiveUpdate. There's no comment on just how many systems were taken down by the botched update, nor what Symantec plans to do for those who were affected.