Personal feedback suggestion
When I get an answer wrong I sometimes make a note of it in my exercise book so that I can keep track of my mistakes in order not to repeat them over and over again. But it is really time-consuming and more often than not I just read the correct answer and hope I can remember it next time (especially during the timed practice.. there usually isn't much time even to read the correct answer :). I was wondering if Duolingo could have a function for that. I'm not sure if it is possible but it would be nice to have a tab up there with "personal feedback" - a list of the sentences I got wrong (the actual mistakes) on the left and the correct translations on the right. Or a list of words that I frequently misspell. I think this would be very useful in the learning process. .
Well, words you frequently misspell are shown in the vocabulary list, by order of word strength. The notepad feature you ask for has been suggested before(search the forum).
In terms of statistics of wrong words, I'm not sure if that will be possible because Duolingo seems to want to shield us from negative feedback, so it hides how bad/good you really are (the whole open vs closed debate).
If you are a software developer or know one, an alternative would be to create an Web Browser Extension for Duolingo. This seems to be the best bet for this personal feedback, and should be relatively easy to implement.