I used Anki to study Chinese for a period of about 10 years. At some point I decided that I wanted to memorize the poker "outs" (probabilities of filling out a hand based on what had currently been dealt). Then I went through a time where I was really busy and didn't study the poker deck for a month (but I made time for Chinese). When I came back to the poker deck, the "spaced repetition" system was completely broken: I had a massive long list of cards that had expired, most of which I'd completely forgotten; but it just kept showing them to me in one giant loop, rather than focusing on a few to actually teach me. And I didn't even have a clean way of telling it, "Just pretend I haven't seen any of these cards at all". I ended up just deleting the deck; that discouraged me from doing anything else I wasn't willing to commit to doing every single day.