I seriously doubt that it was a mistake. This was not a butterfly ballot. I think that it was a message that this particular person thought that nominating Kerry was a mistake.
One of Minnesota's 10 presidential electors broke from the pack and cast a vote Monday for John Edwards, the Democratic vice presidential running mate for John Kerry. The other nine Minnesota members of the Electoral College voted for Kerry, who won the state's popular vote in November.
After the state's Electoral College ceremony concluded, no one stepped foward as the Edwards voter. Most electors chalked the vote up as a mistake rather than a purposeful political statement.
UPDATE: Upon reflection, I didn't make clear that I disapprove of this. I don't want electors to the Electoral College to even think that they have any legitimate basis upon which to modify (even insignificantly) the election results.