As others have done before, I accidentally upvoted a comment that I wanted to flag.
But I am not asking for a way to cancel the upvote. Rather I would suggest moving the "flag comment" to the right margin of the comments.
Currently, on the right margin one gets a link to the little cross in order to delete one's own comment; and on the left we have the up-vote and the flag. I think it would make more sense to have the flag on the right, together with the delete-comment button, and keep the upvote on the left. That way, there is no danger of upvoting when attempting to flag, and in a sense it keeps the positive and negative actions separate: positive action (up-voting) on the left, and negative action (delete, flag) on the right. It seems to me to make more sense.
So I am requesting that the Powers That Be consider moving the "flag this comment" link to the right of the comments.