Statistics. I took it in High school, but it was so poorly taught that I learned almost nothing from it. Now I use statistics every day.

Scripting Languages. I learned Java in High School and rode that knowledge all the way through my CS degree. But when I got a job as a software engineer, Java was among the worst languages available for solving the type of problems with which I was dealing. Looking back at my old code, I could have saved hundreds of hours if I had learned Python instead of Java.

Related, curves I haven't climbed but wish I could/would/intended to:

A European foreign language. I dabbled in a variety of languages in school and settled on Chinese, wasting many years and having little to show for it. If it had been an Alphabetic language (and better yet, a Latin-alphabet language), I'd have a much higher level of proficiency.

A Martial Art. I love the martial arts, but I've never been able to devote myself to any one of them.