It's a weirdly general answer, but I'd say it's when I not only can't replicate something in a field I consider myself good at, but have no absolutely no idea how it works in the first place.

Usually, that provides a nice reality check that no, I'm not anywhere near as good at this hobby/type of work that I thought I was, and a great incentive to learn more about what I'm missing there.

That's always the most humbling thing to me. That not only are there some things that I have no idea how to do, but that I literally don't know the thought process or reasoning behind how those things work in the first place.