Not so much a false positive belief but a conspicuous failure to fill the blanks in the world model: I didn't realize that the night sky appears to rotate until I was a teenager reading Carl Sagan's Contact, and came to the part where the character is watching the sky at night and the stars are described as moving slowly along the night. Then it was obvious that of course that's what happens, but this was the first time I'd seen anyone say this aloud, and before that I would've just assumed without thinking that stars stay in the same relative place all night.
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.