I just upvoted everyone. Then logged out and back in again. And was allowed to upvote everyone again. Hooray!
You've always been allowed to "do" that.
I say "do" as our forums code simply "records your vote preference", and then kicks off a separate task to actually "make it so".
In the end, the last vote wins and gets recorded – as each "per forum post, per user" vote can be expressed only once, but you can change your mind anytime.
Unfortunately, the front-end code isn't currently showing which way you voted, so by tricking things a little you can make it appear as if you managed to (up- or) down-vote somebody into oblivion… but in reality, so long as you're using the same user account to make your "tests", all you've accomplished in the end is to choose whether to upvote or downvote a post.
This is going to be addressed Soon®: we'll be showing a slightly different icon depending on which way you up/downvoted a post (even with JS disabled, obviously), and (while still allowing you to re-cast your vote anytime) the numbers should be quite a bit more clear in the end – as the JS will also be able to take into consideration whether you had or hadn't already up/downvoted a post – and it not being currently able to do so is the source of the "visual bug".
The other "bug" is that we don't currently have a way for you to reverse your vote choice, i.e. if you upvote somebody then you decided that you actually didn't want to either upvote nor downvote the post, you can't go back to "no vote preference given": once up- or down-voted a post, you can only pick which way to vote. We're unlikely to fix this anytime soon, to be honest. Little to gain for a very small corner case.