Prosopagnosia is the inability to remember or distinguish faces. Oliver Sacks recently wrote an article in the New Yorker about how he suffers from this: he failed to recognize a doctor he'd been seeing for many years and he mixed his own two brothers up! Well, nothing like that has ever happened to me, but I do have a terrible memory for faces. At church, work (where I come in contact with 100+ students) and other social places faces just slide by me. I've perfected the art of standing there, smiling, talking to someone who clearly knows me while I wonder who is this person? where did I meet them?
Even worse, I sometimes make up names for people ("she looks like a Gloria") and then I remember the fake name instead of the real one. I'm not sure if this is a side effect of prosopagnosia or just the writer in me taking over.
Think about writing from the point of view of a character with this problem. It's interesting what a barrier it is to basic everyday life.
What I find even more interesting is the people I do remember. You'd think a disability like this would be equal across the board, but it's not. Certain people always stand out, which makes me think that facial recognition is a fairly complex process in the brain. You might call this love at first sight, even though often it's not love, exactly, more like a certain heightened interest. It's the total opposite of prosopagnosia and also a fascinating thing to write about...