When entering an URL for a possible duplicate (when voting to close a question), if the current question's URL is [accidentally] entered, a message "no eligible question found at that address" (or somesuch) is displayed. A more specific message might be clearer. I confused myself with my clipboard and it took a few moments to find out what I'd done.
A bit of a micro-optimization, but probably a useful one.. I made the check a bit more verbose for this specific duplicate case.