Question closed while composing an answer
I understand why this happened, but as a user, it's a frustrating experience. A question was open when I started answering it. I spent about 20 minutes on my answer and went to submit it, only to find out that the question was closed as a duplicate and I couldn't submit my answer. The only notification was that new answers had been posted.
It would be nice if I could still post my answer if the question was open when I started, even if it's closed by the time I'm done. Having one more answer on a closed question isn't exactly going to hurt the site. At the very least, a 'Question Closed' notification bar should appear, similar to the one that appears when new answers are posted.