Undownvoting or the deletion of a downvoted post. There ought to be a "show removed posts" checkbox at the bottom of [your reputation history][1] that will display this.

You can also audit your reputation from [here][2] (though this is considerably more confusing)


  [1]: http://mathoverflow.net/users/6976/mark-sapir?tab=reputation
  [2]: http://mathoverflow.net/reputation