I had a big one regarding psychology after taking an introductory psychology course with the best textbook I've ever read on anything, ever. The textbook was so inclusive, and my recall of it so great, that it quickly became very common for me to shoot down (extremely) basic psychology misconceptions, and I acquired an over-estimated view of my understanding of psychology. The primary trouble was how it only covered a few of the standard biases, like hindsight bias and illusion of control, so I thought I was much less biased about psychology than I actually was...
I also had one regarding knowledge. If you talked to me a couple of years ago, I could have waxed endlessly on uncertainty and the unknowable nature of infinity and all sorts of nonsense, which was only starting to dissolve when I finally discovered LessWrong, after which I realized I had been a complete fool, and it dissolved completely
It's practically cruel of you to say this without telling us the textbook's name. Even after bringing up this downside, I'd want to read it.
A little knowledge is a dangerous thing, not sure what the official name for this particular cognitive bias is (feel free to enlighten me). Probably most of us can recognize that feeling of enlightenment after learning a bit of something new and exciting, and not realizing yet how far it is from the mastery of the subject. I suspect that learning the LW brand of rationality is one of those. (Incidentally, if the words "LW brand of rationality" irked you, because you think that there is only one true rationality, consider how close you might be to that particular summit of Mt. Stupid.) See also the last bullet point in the linked comic strip.
As an exercise in rationality, I suggest people post personal accounts of successfully traversing Mt.Stupid, or maybe getting stuck there forever, never to be heard from again. Did you find any of the techniques described in the sequences useful to overcome this bias, beyond the obvious of continuing to learn more about the topic in question? Did you manage to avoid turning Mt.Stupid into the Loggerhead range?
My example: I thought I was great at programming fresh out of college, and ready to dispense my newly found wisdom. Boy, oh boy, was I ever wrong. And then it happened again when I learned some more of the subject on the job...