Both of the following questions also deal with the same issue, I don't believe this is a duplicate as this question describes steps to create the situation described in the first when not doing a merge (as discussed in the second)
I was able to reproduce this problem myself. Here is how I did it.
I was in the middle of editing a question when it was closed.
I hit the save button and I got the banner saying I could not post because the question was closed.
A comment notification appeared and I clicked on it to look at the comment.
I hit the back button on the browser.
I was now able to hit the save button and post my answer.
Here is the question in question: Dictionary<TKey, TValue> override Indexer and FindEntry
In reference to the grace period I believe this is a bug because the banner came up and I could not post the answer. After moving off the page and coming back I was able to post the answer.
So the bug is one of two things
I should not have gotten the banner when I tried to post the first time
I should not have been able to post when I returned to the page