If you upvote a comment and then try to remove the upvote, you get the following message:
Are you sure you want to undo your upvote on this comment? You will not be able to upvote it again
And sure enough, when trying to upvote afterwards:
What is the purpose of this feature? If I accidentally upvote a comment and decide to remove the upvote, I still want the ability to upvote it if I decide to do so later on.
When voting on questions and answers, you can toggle votes on and off as much as you want for at least 5 minutes, after which they lock. Even after a vote is locked, you can change the vote if the post is edited. Why do comments, which hold much less data integrity value for the site, have more restrictions than questions and answers do? It doesn't make much sense to me.