I notice that once in a while I press an arrow key to get the computer out of the screen saver, and sometimes I accidentally press the down arrow key and downvote a sheep.  I also noticed that in SheepWatcher, the vote keys are F5 and F6 instead of up and down, which gave me an idea.  I think you should be able to change which keys are used for voting, or at least have a couple presets.  That way people probably wouldn't accidentally downvote sheep as much.