Profile downvoting. When someone goes through your entire profile and downvotes everything because they have spite for you. I've had it happen to me and many other prominent users have experienced it too, and you can tell because you suddenly get -1 on all your posts, even ones that couldn't possibly be inflammatory or posts on small communities that have very little activity, and they come close enough together that it can't reasonably be from separate people. Also related: down voting everything in an entire community.
I know internet points mean nothing, and honestly I don't really care, but it's just annoying. Like, what did you gain from doing that when the block button exists?