I love the example of what causes the phases of the moon, because it's not knowledge that most (modern) people have cached, but figuring it out only requires drawing a bunch of picture and asking questions about them like "What would I see in this configuration?".
For my part, I never realized (or probaly just forgot) there was a pattern between the phases of the moon and the time of the moon's visibility until I did this exercise a few years ago.
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.