Did i receive a phony warning from Win. Defender?

Win 8:

                while at mail program (i think -now not sure) suddenly rec'd popup from Windows Defender(?)  (showing as  'defender') listing some ''possible potential' threats that could crash my system & a choice to have 'defender' fix the problem(s) -- i know Windows Defender is turned off when another security system is installed - could that popup have been legitimate ? Do these false things happen?    i closed -out -did a full system scan -nothing found - also did a Super AntiSpyware scan -no problems showing other than a few 'cookies' ..  i'm confused !   thanks so much