Before coming to LW I intuitively believed in the map/territory distinction (physical realism, if you will).
Physical realism is not the same concept as the map/territory distinction. Korzybski who coined "The map isn't the territory" distinction wanted to get rid of discussing "Is X Y?"
Maybe scientific realism? Not sure. In any case, I prefer the original "the map is not the thing mapped" vs "the map is not the territory" as just as potent but free of ontological baggage.
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.