Clicking on the "delete answer" button in the "not an answer" review queue shows a JavaScript confirm with the message "undefined". This message shows before and after a successful delete vote, i.e. I can click on the same button numerous times and see the message.
I looked for errors in the browser console (none) or HTTP traffic preceding the alert (none).
I've only seen this error in the last 36 hours, and it doesn't appear to be an exact duplicate (though "undefined" is common in JS and such messages have showed up in other places in the past).
I am seeing this behavior in Chrome 25 and IE 10.