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.