I just upvoted a comment and was immediately greeted by this alert:
Are you sure you want do undo your upvote on this comment? You will not be able to upvote it again.
When I clicked OK the comment was not upvoted and I could not upvote it anymore.
This behavior seems to be a bug.
Using Google Chrome 28.0.1500.71 on OS X 10.8.4 with an Apple Magic Trackpad.
Edit: This seems to be intermittent. It happens about every three or four times. I could reproduce it at least three times. I could not reproduce it by double-clicking. I don't have problems with accidental double-clicking elsewhere, even on Windows I remote desktop to.