Strengthening skills I haven't learned yet
It's interesting, but it's difficult to practice a skill I haven't learned yet. It seems that Duolingo's Strengthens Skills section tests you with words in the following section once it's available for you to learn. This occurs regardless if you've even looked at the section. For example, once I unlock Basics 2, Duolingo will test me on the words in that section, even if I haven't seen them yet. Shouldn't strengthening my skills be an exercise on words I have already had lessons on, and not on words I haven't ever seen? This occurs only while I strengthen my skills in German
I've seen it - but I view it as a feature, not a problem. But I don't tend to be anal about things like memorizing vocabulary, I figure enough repetitions and it will stick, no matter when I encounter it. Since there is no penalty for a mouseover, I just mouseover. The only time I find it to be a real problem is when the new word or grammar concept comes in a dictation, especially in French, since the spelling isn't as regular there as in other languages, and many words are homophones.
I haven't had this problem, and I would actually recomend that you move this to the toubleshooting forum, as it seems to be a bug. If you click to edit the topic, you should get the possibility of moving it to a different forum.
If you are more specific about which course this is happening in, that will probably help as well.