I just dropped by my reputation activity page and saw this:
The thing is, I hadn't removed my downvote from the related answer (the "Remove HTML tag using jQuery" one it takes me to when I click through) or indeed done anything at all two minutes earlier, nor had the related answer been deleted. Why did I suddenly get +1 rep?
Having captured the above, I then did a rep recalc, and it disappeared. Gremlins?