Many problem in problemset have tags that don't have anything to do with the problem, this is rather annoying, especially if you are trying to find problems for a specific topic. As far as I know anyone who solve a problem can edit the tag, which basicly means that anyone can copy someone submission, and edit the tags. For the problem 1713D - Tournament Countdown form the most recent div2 round induk_v_tsiane has been repeatedly deleting tag 2-sat, and someone adding it back.

Also it seems that solution for this issue is not that hard to implement: either a minimum rating to be able to edit the tag, or leave it just for problemsetters.