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.
This happens to me quite a bit in the iOS app but not on the website.
Additionaly, sometimes words which haven't been introduced are asked for. (In my case, I'm learning french from english. And sometimes an english sentence might show up containing a word which is yet to be shown as "new".)
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.
This does indeed happen, but it's not too hard to look up an unfamiliar word in an external dictionary. Another phenomenon is that the list of strengthened words at the end of a review contains words that were not used in the review.