<p><a href="http://meta.stackexchange.com/questions/1170/should-i-be-able-to-cancel-my-up-vote-on-a-comment/129032#129032">Waffles explained why</a> on MSO.</p>

<blockquote>
  <p>We do not want flip flopping, nor do we want people to un-upvote stuff
  from the past. The only use case is undoing a mis-click.</p>
</blockquote>