Just found a URL error
Let's say you have already passed your first checkpoint. But, if you go to https://www.duolingo.com/bigtest/de/0 (this is the German course, but should work for every other), you can still do the big test, which gives you 50 XP. Plus, I've been told that I learned two skills I had available by that time, but haven't learned yet (in my case Dat. Pron. and Negatives). Luckily for me, they are still untouched, so that was just some weird error. Nevertheless, you (talking to the admins) may or may not want to make the checkpoint test unavailable, but you should at least know about this. And (evil laughing), if you all others want some easy XP, hurry up, this may not work forever.