“Thus, on several occasions I have apologized for almost bumping into a large bearded man, only to realize that the large bearded man was myself in the mirror. The opposite situation once occurred at a restaurant. Sitting at a sidewalk table, I turned toward the window and began grooming my beard, as I often do. I then realized that what I had taken to be my reflection was not grooming himself but looking at me oddly.”
Oliver Sacks, writing about prosopagnosia, a condition that makes people unable to recognize faces. Link is to abstract of article.