Believing this wasn't rational. I didn't take such basic steps as looking up the costs
Believing a proposition X with some probability, without checking it, isn't irrational. I'm very confident that the core of the moon isn't made of cheese, even though no one has ever checked that. The whole point of theorizing is to jump from a limited number of empirical observations to general statements about the world.
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.