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][1], he has indeed received a downvote for it.


  [1]: https://superuser.com/questions/5844/5845#5845