I just got the "You conquered the Italian skill tree!" bug!!!
I just finished the last lesson of a unit in the middle of the italian tree, and at the end, instead of the usual "congratulations you mastered <...>" message, I got the happy owl "You conquered the Italian skill tree!". I am far from completing the entire tree, not even close to the middle, so I think something went wrong somewhere. There is a good chance that there is another tab or window open in the French tree, but I am almost 100% sure that the last language switch I to was Italian.
Interesting - I didn't get any message at all when I did complete a tree, nor anything in my stream.
Happened to me a few months ago (here's the thread about it: http://www.duolingo.com/#/comment/379996?from_email=comment ) but I haven't seen that notification since.