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.
For users answering from a mobile device, there's no client-side restriction in place to begin with, so this situation is presumably even easier to create when using the mobile version of the site.
As a side note, this apparently even works for questions closed due to migration. (Editor's note: This was fixed sometime in 2012.)