.Net 3.5 fails to install on Windows 2012 R2 - KB2966828
Last Updated: 8/21/15Installing .NET framework 3.5 on Windows Server 2012 or Windows Server 2012 R2 normally just requires using the add features function of Windows, mounting your Windows server 2012 ISO, specifying the alternate source (D:\sources\sxs\), and choosing install.
However If you did NOT install .NET 3.5 and you HAVE installed all the available windows updates, then the .NET framework 3.5 feature will fail to install.
The solution is to remove update KB2966828 first, then install .NET framework 3.5, then reapply windows updates.
source:
http://social.technet.microsoft.com/Forums/windowsserver/en-US/ea89a916-e198-4a55-bf3c-5a9029e9e344/cant-install-net-framework-35-features?forum=winserver8setup