I hit c) for the category theory course in my masters. I managed the first half, more or less, but it felt like it was ramping up exponentially; there were too many new layers of concepts all of which were defined in terms of the previous one, and every new layer meant a percentage slowdown in my ability to work with that concept.
During undergrad I'd been at about the 30th percentile, but only the best half of undergrads go on to do a masters (at least at that particular institution). In retrospect it shouldn't have been a surprise that I was towards the bottom of the class, but it was.
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.