The effect you mention can be either subtle or you can be hit over the head with it. For example, I was totally nerd-sniped by xkcd the other day.

It has also happened to me with Confluent hypergeometric functions especially in Abromowitz and Stegun. It looks like Math::Cephes has both the Hypergeometric and the Bessel functions, and even the more useful (to me) modified Bessel functions.

I've learned a lot of things over the years that I haven't used yet. I consider it to be 'backlog' at least until I forget it!