I was trying to vote to close a question1 as a duplicate and, after I entered the URL of the duplicate question and clicked 'Vote to Close', nothing happened (ie., there was no response at all)—despite the fact that I still have 24 votes remaining.
I tried several more times, eventually2 getting an error in the yellow pop-up box:
Question #0 does not exist
I tried this two more times to confirm.
Flagging the post worked fine.
The only way I can reproduce the error box is to step through selecting all of the other reasons to close the question before reselecting "exact duplicate" and resubmitting the form.