I just happened to see the question Is it okay to whistle while at the office? and noticed that this answer was posted 32 minutes ago. However, the question was put on hold 53 minutes ago. How is this possible? Shouldn't putting a question on hold prevent any new answers being posted?
If a question is closed while you're answering on the full version of the site, you will receive a notification that the question was closed and the 'Post Your Answer' button will be disabled. However, this is only a client-side restriction, so it is possible in some cases that this process fails.
When it does, the backend allows for a grace period of around four hours (previously there was no time limit). Presumably the client-side restriction did fail in this case, and since the answer was "in-flight", as Jeff describes it, the server accepted it despite the question already having been closed.