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.
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.