Running OS X Yosemite, but About This Mac shows Version 10.90
Download the latest Yosemite installer from the Mac App Store but do not install it.

Copy following in to your Terminal to verify the downloaded Yosemite file is correct:

shasum /Applications/Install\ OS\ X\ Yosemite.app/Contents/SharedSupport/InstallESD.dmg


Wait for a minute for it to calculate. The result for the previous version should be:

eebf02a20ac27665a966957eec6f5e6fe3228a19  /Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg


And for Yosemite Download as of Nov. 27 the result is:

a673c2c6d967f4da2934b7d6cf3736936970b194  /Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg


Proceed with Install.