Sometimes when learning news terms and selecting an option under "I know this" (I.e. a bit, well, or very well), the term still shows up in the accompanying quiz. During the learning portion before the quiz, the term will properly get replaced with a new one, but the old term shows up instead of the new one afterwards in the quiz.
A few notes that may help:
Happens both for kanji and vocabulary
Only remember seeing it occurring for 1 word per quiz max (possibly just by chance) and doesn't always happen
