Sorry if this has already been mentioned. I am occasionally (about once a week) getting a -1 for days studied. Not a serious problem, but a strange one. Not sure if it is messing with my review stats, but thought I'd point it out.

Thanks for letting us know. I've seen a few reports of this and have opened an issue for it. The stats page will be getting an overhaul in the near future and this will get fixed.