December 2014 may turn out to be the worst month in Windows patching history -- and that's saying something. KB 3025390 has hit the Automatic Update chute. Unfortunately, the link in the Windows Update description doesn't work, but you can find detailed information at support2.microsoft.com (note the "support2" in the link).
Here's what that KB article says:
You install MS14-080: Cumulative security update for Internet Explorer: December 9, 2014 (https://support.microsoft.com/kb/3008923) on a computer that's running Internet Explorer 11 or the Internet Explorer 11 Web Browser control. However, after you do this, you may experience unexpected behavior when you interact with sites that use one or more web application modal dialog boxes. Any data or information that's provided in the modal dialog box may not be returned to the application window or to the dialog box that created the data or information. Therefore, the application that created the dialog box may exhibit errors or lack specific functionality that was dependent on that dialog box data.
German sites report that the patch appears in the English language, though their patches normally appear in German.
We are aware of some reports of functional issues on sites that use nested modal dialog boxes on Internet Explorer 11 that occur after you install this security update…
We are aware of some limited reports of Internet Explorer 9 crashing after you apply this security update.
Microsoft is researching this issue and will post more information in this article when the information becomes available.
Many people haven't been able to find the KB article, and they're cautious about installing a patch simply because it magically appeared in Windows Update, with no explanation -- very sloppy, Microsoft.