I vaguely recall believing when I was young that there were no real bisexuals, just gays in denial about it.
I used to think acne was unrelated to diet (other than perhaps via direct facial contact with grease).
When law enforcement first started being equipped with tasers, I thought this was a good thing, because they would use nonlethal force on occasions where they would previously have used firearms. It turned out that police continued to use lethal force as before, and instead used tasers in situations where they might actually have talked people down in the past.
Ironically it's much more likely that gays are just bisexuals in denial.
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.