Prosopagnosia is also known as "facial amnesia." The ability to recognize faces is impaired. For me, I didn't realize I had it until someone was saying to me, "I picture his face everywhere I go." I thought about it and I couldn't even conjure up a speck of my own face, my father's face, my mother's face, my son's face... I literally had no faces in my memory bank. All my life, I had learned to notice something odd about a person to recall them, like their voice, their hair, a scar that intersected their brow, their beady eyes....
But, if I were to run into someone I know very well in a place I did not expect to be meeting them, I would not be looking for that characteristic and therefore would stare blankly at them. I hurt more than a few people's feelings that way. I learned to recover and explain it away, but I didn't realize others could actually recognize whole faces, even picture them in their minds.
I am not alone in this oddity. Brad Pitt recently admitted to the issue and having some of the problems I faced to, people thinking he was snobby or didn't like them because he could meet them and not acknowledge them. This is one of the reason I so dread things like conventions where I must meet people and then run into them later at the event and have no clue who they were. I have been known to expect to meet someone I know and go towards someone with similar features to hug them. Luckily, keen friends with me can warn me of my ill fate.
Recently, after having realized my oddity, I was talking with my sister when I recalled her aversion to making friends and being antisocial. I never really understood why she hated meeting people so much. Then, I thought to ask her about faces. She closed her eyes, could not conjure a single feature of any face including her own. Each person adapts to the condition different ways, her way was to simply not have to recall anyone by not knowing anyone. Oddly, my case made me quite content to do public speaking and modeling where I did not have to be learning people one-on-one, but en mass.
Others with the condition; Jane Goodall, Oliver Sacks (famous neuroscientist), and Paul Dirac (physicist and Nobel Laureate).