I have not seen this word before
Every now and then, a word appears and Duolingo (at least in the iOS app, not sure about web) believes that I have seen it before but I have not. Sometimes I cannot even peek and have to guess. It's very rare.
I've found this happens more and more the farther you go. One common trend I see is a word being used in a totally different context, say as a verb instead of a noun, and it's surprising how often the two are unrelated (you can't just form/unform the gerund or infinitive).
There also seems to be a general issue with verbs not showing up/being completely off, but that might be an issue with the Italian module as it's relatively new.