Say you're browsing the forum on a mobile device and your finger accidentally brushes the upvote button.
If you try to reverse it by tapping the downvote button, then you have downvoted it, rather than un-voting.
In reality, I'm neutral about the topic... but because of a momentary touch of the screen, I'm now forever either + or - about it.
Irrevocable actions (with no apparent good reason for being irrevocable) are a sign of bad interface design or implementation. Can this be fixed?