Before coming to LW I intuitively believed in the map/territory distinction (physical realism, if you will). After going through the countless arguments of the type "Is real?" (where can be qualia, consciousness, wavefunction, God or what have you.) I gradually came to the conclusion that the term "real" is both misleading and counterproductive. If a sentence (excepting mathematical statements) cannot be rephrased by replacing "real" or "true" with "accurate", then it is meaningless.
Up next: stop believing in using parentheses so much.
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?"
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.