I've put a question on hold, nevertheless a user was able to post an answer later on:
Seems like a bug to me, but maybe I'm missing something here.
It's indeed possible to answer a question which is put on hold, if you're still on the question page. You'll get a banner "this question is now put on-hold" and this will disable the 'Submit' button, but it's relatively easy to re-enable it via the developer tools of the browser. The server will still accept new answers for a few hours or so.
See also this Q&A on Meta Stack Exchange.