One of the slides of my physiology 102 course consisted of the claim that women blink twice as much as men. I put that as a fact into Anki. Now it turns out that when I google around it isn't true.
In general the heuristic of believing things I'm told in science university lectures isn't completely bad, but it seems that there a category where lectures want to mention interesting facts and then tell students interesting facts that aren't necessarily true.
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.