There is a disease, typically brought on by a minor stroke, called prosopagnosia, in which the victim can no longer recognize faces. These patients can tell they are seeing someone’s face, but they cannot tell whose it is, even if the person is a close friend or relative.
As many as one out of every 50 people have mild prosopagnosia, in which they have more trouble recognizing faces than everyone else. You may have it if you fail to recognize someone you know in a new hat, or having shaved off a beard.
If you have a new friend who fails to recognize you, don’t get mad, get even. Wait, that’s not right. Just realize that the person may have prosopagnosia.