Extra Lesson I Didn't Learn
I just finished Adverbs and the next 2 chapters were unlocked with the first lesson of each already finished.
Something similar happened to me in the French tree when I got to the modal verbs lesson. I contacted Duolingo, but never heard back. When I finally got around to looking at the lessons I had "completed" I could see that they were sentences that had appeared in previous lessons (which sort of stands to reason with modal verbs). Take a look at the lessons and see if anything looks familiar to you.