Before the “quiz only readings I’ve studied in vocabulary” option existed, I manually set off readings I hadn’t yet seen on a bunch of kanji. Then when that option came out I turned it on. Now when I finally hit a word with that reading, they’re still stuck off, and clicking them shows a popup saying I’ve marked it not to study but no longer has the checkbox to undo it.
The most recent one I remember: the つの reading of 角.
