How was an answer posted AFTER a question was put on hold as a duplicate?

Question

See here:

Way to quickly select random objects with variations in foreground/background

Closed as duplicate nearly 20+ minutes before an answer was posted.

How'd that happen?

The answer was started before the question was closed you say?... Oh no....

Typically.. if you start an answer and the question gets put on hold as you fill in the answer, as soon as you click "submit" you get a message stating answers can't be submitted. (Has happened to me many times. :))

Also note that my rep is high enough that if the user posted an answer... and deleted it... (which they commonly do) I would have seen the deleted answer when I closed the question. They could have then gone and edited the deleted answer and undeleted. But I saw no such answer when I closed the question.

I'm baffled as to how that answer got posted.

Answer 1

There is actually a grace period of about 4 hours after a question is closed where answers will still be accepted (I'm not sure if the 4 hours is still accurate, but the grace period certainly still exists). If you're on the website there will be a banner telling you the question is closed and the button to post the answer will be disabled, but these are client-side restrictions*.

I believe the mobile site and the apps will still allow you to post an answer unless you explicitly reload the question (they certainly did at some point, not sure if it has since changed). It's also possible the client-side restrictions on the website failed for some reason.

See e.g. How was this answer posted after this question was closed? (and the related/linked questions thereof)

^{* which means it's possible to open your browser's developer tools and re-enable the button to post the answer.}