Prosopagnosia is a perceptual phenomenon in which a human tends to have difficulty recognizing other human faces. This definitely falls into the category of things that I was kind of amazed to learn about (as in, "you mean there's a WORD for that?!") as well as things that I thought "everyone" experienced until I learned otherwise.
In my case I am capable of learning to recognize my fellow apes once I'm very familiar with them, but when someone I see only casually or infrequently gets a haircut or something, I'm liable to not know who the heck they are unless they happen to tell me.
I also used to run into amusing situations when I worked at my last job, as there were some people I only EVER saw in the lab or manufacturing areas. Everyone in those areas was required to wear blue lab coats, and for some reason my brain ended up integrating "blue lab coat" into whatever algorithm it used to recognize those folks. Hence, if one of them randomly walked up to me in the grocery store, initially I would find myself going "gah, who IS this person?" and then finally having to just come out and ask them who the heck they were.
Oh yeah. And when I first saw "Star Wars" as a kid I knew I really LIKED it, but I could not for the life of me tell those two white guys (Han Solo and Luke Skywalker) apart...not until I figured out that their SHIRTS were generally a different color. Seriously. I also spent several years of my childhood under the impression that removing/replacing my glasses was an awesomely effective disguise, seeing as I CLEARLY looked like a completely different person based on whether I was wearing glasses or not.
But! Regardless of whatever issues I might have with HUMAN faces, apparently this doesn't carry over to feline faces. Because every cat I've ever seen looks different from every other cat I've seen, and this is almost always immediately obvious to me. (Any neuroscientists reading this want to take a crack at explaining this one, I'd be very curious to get your thoughts!)
Case in point: two of the three ex-feral littermates sharing my home happen to be blue mackerel tabbies. And I've noticed that a lot of humans persist in classifying cats based almost solely on their coat pattern/color. Several people have, upon seeing Coraline and Brodie, asked me "...but how do you tell them apart? They look like twins!"
Um...no they don't. Not to me at least. They're siblings (brother and sister) so there are certainly aspects they share in common. But they do NOT look anywhere near identical.
Heck, they're not even the exact same color (despite falling generally into the "blue tabby" designation). Brodie is a lighter/softer grey, whereas Cora is "higher contrast" and has some areas of lighter fur that range almost toward a brown-tan color. You can sort of see this in the picture below (in which the tabby siblings survey the new shelves I just put up in the spare bedroom recently):
These two kitties are also quite different in size, though admittedly that's a lot easier to perceive in person than it is in photographs. Cora is a little compact kitty with a "roundness" to her (which isn't anything to do with weight...she's only 7 lbs). She also has this rather unusual fur that isn't quite long enough to be "medium" but which is extremely fluffy and kind of sticks out rather than flopping over (if that makes any sense). Brodie, meanwhile, presently weighs 14 lbs and is much longer-bodied than Cora.
This picture (above, in black and white), with Brodie on the left and Cora on the right, shows some pretty dramatic differences in how their faces are shaped. First off, there's the ears: Brodie's are more triangular, whereas Cora's are more oval-shaped at the tips. Cora also has kind of a heart-shaped face, while Brodie has a pointier chin and overall a more triangular face (with a squarish jaw...he's got some Maine Coon in there somewhere I think, despite his very short, fine hair).
...and here we have a really clear view of their differing profiles. (And it's just the camera angle making Brodie, on the left, appear smaller than Cora, on the right...again, he's a lot bigger than she is IRL). Brodie has a rather long head and muzzle, almost to Oriental/Siamese proportions, whereas Cora has a shorter muzzle and a little vaguely up-turned nose. Again, very different-looking IMO.
Of course their personalities are also quite different -- you'd NEVER mistake them for the same cat if you actually knew them. But even on the level of "mere" appearance, to me there's just no question they're not identical by any stretch of the imagination. They're both beautiful, certainly, but not in the exact same ways!