"Strengthen Skills" gives words from next lesson (that I haven't learned yet)
This has happened to me a couple times, but I'm not entirely sure how to reproduce it consistently. It just happened today with the latest version of the Android app.
I use the Android app almost exclusively for practice. This scenario occurs when all my previous skills are "gold" and my next lessons haven't started yet (0 out of n in all the next open skills). I usually do a "Strengthen Skills" at the beginning of my practice sessions, even when everything is gold, and sometimes it seems a few words from the next unfinished lesson get added to my weak words list, even though I haven't officially been introduced to them yet.
Today I had everything up to "Adject. 1" in Spanish in gold, and I did a strengthen skills. It gave me some sentences with están, aparecen, and sirve from the first lesson of "V. Pres. 2" even though I hadn't completed that lesson yet.
Yes, me too in Spanish. Plus, I foolishly bought the idioms bonus skill only to find that I was being asked to translate complex phrases that I had not been taught. I'm not sure how a learner only just introduced to a vocabulary of less than a hundred words and only one tense could be expected to guess at the Spanish for 'Out of sight, out of mind'. The only way for me to complete this skill (and there are three sections) would be to research out of DuoLingo for each question. However, I think I'd still struggle with the 'type what you hear' tasks.
It works for the translations from Spanish to English but not the other way round. The worst ones are the 'type what you hear' and then comes a complex phrase you've never heard before. I've stumbled through the three lessons in idioms with a combination of guesswork and looking up in the dictionary, but I would say it had harmed my learning rather than taught me anything. I love the rest of the course, so this seems a bit of an aberration.