On the above question, I upvoted a couple of answers with 0 vote count and the count was not incremented, but the uparrow highlighted.
Being confused by this I clicked the upvote again and the count changed to -1.
That's probably just someone else downvoting at the same time as you upvoted. When your upvote was in place (arrow highlighted) the two cancelled each other out, leading to a total of 0. When you then undid your vote, it went down to -1.
EDIT: In particular, if you mean Jonathan Sampson's answer, he has indeed received a downvote for it.