I can't really think of anything I truly regret. Smoking weed has become a bad habit and it affects my school and sports life, but I enjoy it at the same time, and can't really consider it a regret.
The thing I regret the most is not taking care of my first car. It was a beautiful boat of a car, a `92 lincoln towncar. It was the perfect hotbox-mobile. I was an idiot and didn't check the oil frequently enough and I blew both the head gaskets.