Let's start off with an example: http://www.devmaster.net/forums/showthread.php?t=12007
Scenario: a user uses the search, finds an old thread and starts reading, sees a post that demands his reaction, forgets he was reading an old thread and thus kicks the thread into oblivion. The thread is now on top of the recent posts list and other people start to reply, unaware of the fact that the thread was accidentally kicked and that most posts are old and outdated.
A dutch IT-related forum I frequently visit has the feature that it hides the quickreply box under a thread when a thread is older than a certain threshold. Replying is still possible, but it is just that quick reminder that the thread is old that prevents people from accidentally kicking it.
It's not that big of a problem really, but the fix is technically very simple and can avoid a lot of confusion