I have this userscriptthis 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.