How long do you think you had the wrong belief? Was it just something that happened in that moment or did you carry that believe around for you for longer?
Just that moment. I definitely didn't follow any of its implications. (Other than "if I say this then people will react as if I said an obvious true thing.")
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.