I found a couple of bugs that you might want to have a look at.
A couple of weeks ago I started redoing all the lessons in each unit ( I keep notes and I am doing the units, and the lessons in the unit, in order).
I had to stop for a couple of days because of all the new lessons on the French tree, and now that I finished the new lessons, I started again.
The two bugs are the following:
1) I occasionally find new words in some lessons, and even when I have 3 hearts left at the end of the lesson. Some of the words that I remember seen, and that are not in my vocabulary list: agriculture, afin, fiancé, sauce ...
2) After finishing the new lessons ( I got my owl back ;-) every time I finish the last lesson of the unit I get again the "Congratulation" pop up window with the Duo Owl congratulating me for finishing the entire skill tree. I guess there is some bug in the checks that ignores the fact that I already finished the tree before.
Nothing very serious, I would really like to have all the new words on my vocabulary so that I can practice them occasionally but it is not a huge deal.
Thanks for checking!
Sometimes the server is kind of slow, you might have to wait for a minute or two. I have seen this kind of delay too. I used to think that it was my browser, and every time I would fail in a lesson more than two times in a row, I would close the window and continue in a new one, I had the impression that they were somehow correlated because most of the times that the page seemed stack it was in this type of situations. Once or twice I forgot the window open, and it eventually moved on to the next page.