After trying to install .net 3.5 via the program and features (see below). I was getting unable to install error 800F0906
I am using windows 8.1.
Found link that identifies two Windows Updates as the problem if DISM and the offline installer do not work. Uninstall KB2966826 and KB2966828, then try installing 3.5 from programs and features. It worked for me.
By Chris Wardario