Happened on this question; https://meta.stackexchange.com/questions/128852/auto-redirect-to-ban-page/128853#128853
I composed an answer while on the mobile site, and submitted it. It submits, except that I see a 'closed by .... 7 mins ago'. Which is strange, cos closed questions aren't receptive to answers.
Pretty big loophole, it seems.
Update: (credit @lunboks) It seems that this is a client-side restriction, which is disabled on mobile. It can most probably be bypassed even on the full site using Chrome insprctor and/or Firebug.
Even though I'm an amateur programmer, I do know that such stuff done on the client side is a Very Bad Thing. I say that there should be an extra server-side block. If the poster gets past the clientside block via mobile site or other tomfoolery, he should get the page reloaded with 'Sorry, the post is closed &c'(iirc a similar thing happens for deleted posts). Try to keep the attempted answer somewhere there, since they may want to copy their hard work.
The possible dupe talks about a grace period, but that's inconsistent with what the main site does(disable answer submit button on close--via websockets I guess)
It seems that if you wait long enough, trying to hack-post on a closed question gives you this: