I think it was trying to learn how to make a CSS-only drop-down menu -- especially multi-tiered.

Even though that's when I was first learning, I'll admit I never totally conquered that one. As much experience as I've had with CSS over the last several years, I haven't had to build one of those menus in a while, which kind of means having to do it again will be frustrating again.

Or I can cheat and use whatever CSS framework I'm using at the time. 😏