False belief: That in the U.S. deaths by firearm are generally homicides, not suicides.
I would have expected accidents to lead that metric. A quick check of the actual data says it's negligible. Time to rescind my support for gun lock laws (except perhaps to reduce the likelihood that people purchase guns in the first place).
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.