I was told that anyone who goes to college is a sheigitz by my 6th grade Rebbe. I went home that night and told both my parents what my Rebbe said about them (I didn’t know about Roshei Yeshivos such as R’ Hutner back then who had gone to college).
When I was 19, the Rosh Yeshiva of the yeshiva I was in at the time told me that if I left the yeshiva and went to college, I was throwing my life away, and there was no way to stay frum in college.
It turns out he was partially right on that one. While in school, he actually contacted me to encourage me to leave, saying I was only harming myself. I decided that if I was doomed anyway, I might as well enjoy myself on the way. Only later (while still in college!) did I decide that despite everything I was taught, he was probably wrong. That was when I became frum again.
Those are a few examples of things I was taught that were wrong.