Have you ever had no recognition of yourself staring into a mirror?
I was walking in a strange building that I did not know had mirrored walls, I person looking directly at me was walking directly to me - I was about to fight my reflection and then realized it was myself.
Some people have a condition where they can not recognize faces, even the faces of friends they have had for years. Very few people are known to have this condition but with more publications on the condition, more and more people are coming out and realizing that they have it.
Sure, I've looked into the mirror before and "forgotten" that it was me...it usually only happens when I'm under enormous stress or severe depression. I think it's natural to get the feeling a few times. If you start experiencing it frequently though, then something might be wrong.