Ask HN: What are your risky knowledge investments?

In _The Pragmatic Programmer_ by David Thomas and Andrew Hunt (20th Anniversary Edition), the authors compare maintaining technical knowledge to investing. In particular, they suggest when learning new skills that it's good (sometimes) to "By low, sell high." Here's the quote:

> Learning an emerging technology before it becomes popular can be just as hard as finding an undervalued stock, but the payoff can be just as rewarding. Learning Java back when it was first introduced and unknown may have been risky at the time, but it paid off handsomely for the early adopters when it became an industry mainstay later.

What are your risky knowledge investments?