Oh, that's a good example too for the main thread! I don't remember exactly when I learned that the "eccentric orbit" theory of seasons was wrong (some time around grade 8), but remember how it happened---someone claimed that seasons in South America are flipped from the Northern Hemisphere, and I thought "that's obviously wrong!!", but asking around a bit other people confirmed it.
I guess this is also very similar to Folk Theories of Heat Control---you tend to end up with the simplest model which explains all relevant observations. As long as you only deal with one hemisphere, the eccentric orbit theory mostly works, and it's much easier to remember.
I wonder what this says about how seriously we should take Occam's razor.
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.