I once spent about 20 minutes in class trying to justify the claim that a cubic meter of water weighs one ton, not 100 kilograms.
As you may note, this is not a false belief; it's relevant because I spent a significant fraction of this time wracking my brain trying to jog it free to see if I was being dense (pun intended) like you're describing (I have pretty strong reasons to believe I didn't remember this incident backwards as a case of self-justification by retcon)
You just made me sufficiently confused to quickly google the volume of a cubic meter and the weight of a litre of water. I suspect this is a good habit to reinforce but I can't help feeling silly.
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.