I'm afraid this question may be viewed as primarily opinion-based but keeping in mind all the marvellous teachers in this community, many of whom I know to be active in the Duolingo forums or Facebook group, I think there's hope for factual, objective answers.
I am wondering whether there are some repeating error patterns (misuses of words, overuse of certain grammatical structures, etc.) that can be traced to be common to people who learn the language in that specific course. This is in no way meant to be a critique of the course in any way, more like a question on what to pay extra attention to, finding my way through it myself. I know there are things the Duolingo method itself makes hard to transmit directly.
One misconception I know I had (and one that I have seen in others, too) was thinking that -u is only to be used in imperative (including "let's") or questions ("shall we") or subordinate clauses, which caused me to be misled in this old comment, for example. So I'm looking for other things that the experienced speakers often find themselves correcting in others, if any generalizations in this direction can be made.