Skill leveling up without being touched.
When leveling up the Pos. Pron. section in German, the Nom. Pron. section also levels up at the same time despite having different content.
By the time I moved on from Pos. Pron. I needed external help to figure out what was going on in Nom. Pron as it was assuming I already knew the new words.
It's a known bug. No idea if it will help to report it to DL again, but here's the bug report form: https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-
It is a bug in many courses from the start of the 'crown method' (spring 2018).
And, as usual from spring 2017, Duolingo's staff does not solve this bug either.
https://forum.duolingo.com/comment/29974297 shows an overview of other false linked skills in the course "German for English speakers".