Found a user with negative xp!
I was looking through the comments of the new events update, and saw moderator Usagiboy post a link to this discussion. I looked at the person's profile (here) and they have negative 42 xp in German. Weird right??
They're level one with 102 xp to go to level 2, with the maximum being 60!! What is going on here?
I'm only speculating, but I'd guess he got the 42XP by rating sentences in immersion, then reset his tree (removing all XP), then un-rated the immersion sentences (removing the 42XP previously gained). I think the way in which Duolingo used to work would have allowed this to happen.