IOW, not being able to recognize when there’s something wrong. Originally coined for people who didn’t realize they were partially paralyzed, etc. but in this series of essays also refers to not being able to recognize your own lack of knowledge.
I firmly believe that it’s important to remain open to the idea that there’s lots of stuff that you have no idea is even something you should be asking about. (Whew, that was a roundabout way of saying it.)
Rumsfeld got a lot of mockery for saying it, but it really is best described as the “unknown unknowns.” That’s what’ll get you every time. I actually used the whole quote in a presentation several years ago, about a web assessment project. Because we (I had a cross-departmental team!) discovered answers to questions that had not even occurred to us. That’s why I love (and hate!) usability testing. You have this script of questions that you want to have answered, but people pop up with all kinds of craziness. And that’s a GOOD thing!
I haven’t finished reading the essays, only just got done with the first one (ha!), but it reminds me of my main criteria in hiring an assistant, back when I had such a thing. First of all, they should be ready to learn. I’d've rather trained someone who didn’t know much but was open to learning over someone who thought they knew it all — even if they were pretty knowledgeable.
I try to keep that attitude in myself, because I believe that’s what makes it possible to even BE a web generalist. I don’t know if I always succeed, but I keep trying.