Microsoft has once again discovered a critical issue in one of their updates. In this case it’s the (2010) October CU for SharePoint 2010.

more info: http://blogs.msdn.com/b/sharepoint/archive/2010/11/05/critical-information-about-the-sharepoint-server-2010-october-cumulative-update.aspx

Quote:

Microsoft has discovered a critical issue in the recently released October Cumulative Updates for SharePoint Server 2010 and Project Server 2010, and we have removed the files from download availability.  If you have already downloaded the CU, do not install it.  If you have installed the CU, please contact Microsoft Support for assistance.  We will be posting additional information about the issue here as soon as we have it, and will make the Cumulative Update available for download again as soon as the issue has been fully resolved.

Here is a follow up with more information, details and a workaround:

http://blogs.msdn.com/b/sharepoint/archive/2010/11/06/details-and-workaround.aspx

Update: Updates now republished: http://blogs.msdn.com/b/sharepoint/archive/2010/12/01/october-2010-cumulative-updates-for-sharepoint-amp-project-server-2010-republished.aspx