Today, my downvotes stopped working. I can press the buttons, but after refreshing downvote marks disappear, while upvote marks stay (which means that the scripts at least partially work; I also tried another browser). No message to the effect that I'm not allowed to downvote ever appears. I suppose I could've made 25000 downvotes, counting 10x post downvotes, to hit the 4*karma limit, but it seems unlikely, and there should be a message if that limit is reached in any case.