My biggest mistake comes to a tie between these two:
When I was in middle school, I was a pretty lousy algebra student. I just didn't apply myself, really. When it came time to choose classes for my first year of high school, I heard that geometry was the standard 9th grade math class. I had heard all kinds of horror stories about how hard geometry was, and since I had done so poorly in algebra I didn't think I was ready, so I took algebra 1 again. That was a big mistake. Geometry was probably the easiest class I've ever taken, and I scored perfect 100s on tests after barely paying attention (which is a bad idea, of course, but with scores like that I just fell into the habit). I also call a failure on the behalf of the school system, as they didn't tell me that such a decision would screw up my entire high school education. However, I have a pretty good way to solve this problem: after my senior year I'm taking a calculus course at a college over the summer, so I'll be prepared. Still, it caused a big mess.
My other biggest mistake was joining the Boy Scouts of America. I was in elementary school when they came to preach their lies to us. They said how much fun Boy Scouts was, and in my naivety I signed up. I don't know if its different in other parts of the country, but the local Boy Scout troop which I was inducted into was essentially a redneck correctional program. Tired of yelling at your kids? Put them in Boy Scouts! They have professional kid-yellers. I wanted to quit by the first meeting, but my father wouldn't hear any of it. I don't blame him, he was just trying to be a responsible parent and teach me some valuable skills, whether I liked it or not. My experience in the Boy Scouts of America were easily the worst years of my life thus far. It got so bad that I sometimes contemplated deliberately injuring myself just bad enough so that I wouldn't be able to go on camping trips. There were a few consolations to being in Boy Scouts, though. For one, getting out of Boy Scouts provided a huge consolation to my parents' divorce, and it has taught me patience. There will be times in your life when you must suffer for an extended period, but if you persevere you will escape these times.