A new reply to an old thread/topic will (necro-)bump the old thread back to top of recent activity. This may be totally valid and intentional, but the poster should be aware of this and if the current reply is beneficial to the old thread.

Users who are about to reply sometimes aren't aware or pay attention if their answer is of any relevance to an old topic. Does it make sense to reply regarding the past timeline? Are users still active to whom you respond?

It's easy to oversee time and date and if participants of a discussion thread are still active (the BPIP browser extension helps a bit to spot inactive users where a response may not make much sense or you can't expect any more answers if you ask inactive users).