Disclosure: I build VibeLing , a vocabulary app. This post is about the scheduler inside it — what it does, what we deliberately didn't build, and the thing that broke once real people started using it. Every spaced repetition app has one function at its core: given a word and everything that has happened to it so far, return a date. Show it again on that date. Everything else — the card UI, the audio, the streak counter — sits on top of that one decision. We got that function wrong in a...