This has been bothered my for a while. And a related discussion is in metaSE, while the post is tagged with [status-declined] and the community seems to disagree with it.
Sometimes by accident I up-voted a wrong comment. Sometimes I misread the comment, and I want to undo my decision. (My fault, of course!) Sometimes I just want to flag the comment instead of upvoting, but after the upvote we can't even flag the comment! That really makes me hesitate to use the functionality of upvoting and flagging. If we can cancel an upvote in a reasonable amount of time (just like the upvotes for questions and answers), it would represents the community's opinion much better.
Should we have the ability to cancel a comment upvote?