This question was locked by Mark Henderson 3 hours ago (as I type) but the last answer was posted 2 hours ago. How is that possible?
There's a few of these corner case race conditions - I've been able to post answers to closed questions more than once.– voretaq7Dec 3, 2012 at 16:27
I have this userscript installed. Looking at the question's timeline (no rep restrictions) it can be seen that an answer was posted to it at 03:39 which will have rolled it past the front page. MarkHenderson locked it at 03:46 and a new answer was posted at 04:07 so something odd has happened.
The answer to This question on mSO may hold the clue. Part of the lock is a client side restriction that can fail and if it does then the system provides for a grace period for people to post answers.
When I look at it, I see two answers posted 3 hours ago.
I'm guessing that after those two answers were posted, Mark saw it on the active page and locked it.
There are places in SE where time is given only in hours e.g. "Vote again in 2 hours" when it's 1 hour and 1 minute until a new SE day starts. So maybe it's something to do with caching messing up the timestamps on those two posts?
If so, it should go away after a while if you refresh.