Difference between practice from unit and from vocabulary
I can't be 100% sure but it seems that algorithms of choosing sentences given in a practice session launched from a unit and from the vocabulary section of that unit are different.
When I hit "Practice" here: http://duolingo.com/#/skill/de/Verbs:-Present-2 I get a lot of words learnt long ago. They are even shown with full progressbars as they are strong. New words are rarer.
When I click the "View full vocabulary from Verbs: Present 2" link and hit "Practice weakest words" here: http://duolingo.com/#/vocab/e43e0acd1f6d7701daea6702196a9f88, I get just what I need: a good practice of words that are really new to me.
Is this difference intended? Maybe the "Practice" button in a unit does not focus the practice session on new words but on all the words including old ones? Or maybe it's all in my head :)
By the way, there seems to be a bug with vocabulary practice: http://duolingo.com/#/comment/206640. If not for that, I would use vocabulary practice all the time because I need to train new words and not the old ones.