I think this was a great idea for a post. If LessWrong rationality is worthwhile, then it ought to get lots of replies on concrete facts - not moral preferences, theology, or other unproveables.
I used to believe that embryos pass through periods of development representing earlier evolutionary stages - that there was a period when a human baby was basically a fish, then later an amphibian, and so on. I believed this because my father told me so; he was a doctor (though not an obstetrician), and the information he had given me about other subjects was highly reliable. Most knowledge is second hand - it was highly rational for me to believe him. I now know (also second hand!) that Haeckel's ideas were debunked a long time ago - although they might well have been in a textbook when my father was at medical school.
To me, the lesson is trust, but verify.
I only recently realized that evolution works, for the most part, by changing the processes of embryonic development. There are some exceptions-- things like neoteny and metamorphosis-- but most changes are genetic differences leading to differences in, say, how long a process of growth is allowed to occur in the embryo.
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.