GuacAMole
This started as a freeCodeCamp Whack-a-Mole tutorial and immediately went off the rails - in a good way. I followed the core idea just long enough to understand it, then twisted it into something a little dumber, a little funnier, and more my own. It was one of the first times JavaScript really clicked for me beyond just following steps, and it set the tone for how I like to learn: build the thing, then mess with it until it breaks or becomes interesting.