I logged in earlier and wrote out a reply to a message in my inbox but it wouldn't go through when I hit send. After waiting and retrying I investigated further and discovered that the thread had been locked. I then checked the Modlog to see if it had been locked whilst I was typing and discovered that it had been locked 12 hours earlier.

Why can I click on the reply button and open up the reply box on a locked thread when viewing a post in my inbox?

When viewing the thread directly the reply button and box are available but you cannot type in the text box so I suppose I'll be grateful that not everyone who attempts to interact with a locked thread will have their time wasted like mine was.