We've just switched on a new forum feature. It's a "Thanks" button on posts, where you can thank a user for supplying helpful info, or use it as a kind of like/+1 button to say you agree.
If you accidentally thank a post that you didn't mean to, then you can click the button again and it will turn off your thanks. When you've thanked someone the button turns from a thumbs-up into a thumbs-down - this doesn't mean you've given them the thumbs-down, it's the opposite, it's showing that if you press it again it takes away the thanks (but doesn't show up that you've thumbs-downed the post).
See the screenshot below where I've circled the thumbs-up "Thanks" button in red for one of Doug's posts.